Wikipedia

From Wikipedia, the free encyclopedia

The logo of Wikipedia, a globe featuring glyphs from several writing systems

Type of site	Online encyclopedia
Available in	303 languages
Owner	Wikimedia Foundation
Created by	Jimmy Wales, Larry Sanger
Website	wikipedia.org
Alexa rank	5 (Global, January 2019)
Commercial	No
Registration	Optional
Users	>295,987 active users and >79,803,732 registered users
Launched	January 15, 2001; 18 years ago
Current status	Active
Content license	CC Attribution / Share-Alike 3.0 Most text is also dual-licensed under GFDL; media licensing varies
Written in	LAMP platform
OCLC number	52075003

Wikipedia (/ˌwɪkɪˈpiːdiə/ is a multilingual, web-based, free encyclopedia based on a model of openly editable and viewable content, a wiki. It is the largest and most popular general reference work on the World Wide Web, and is one of the most popular websites by Alexa rank. It is owned and supported by the Wikimedia Foundation, a non-profit organization that operates on money it receives from donors.

Wikipedia was launched on January 15, 2001, by Jimmy Wales and Larry Sanger. Sanger coined its name, as a portmanteau of wiki and "encyclopedia". Initially an English-language encyclopedia, versions in other languages were quickly developed. With 5,790,638 articles, the English Wikipedia is the largest of the more than 290 Wikipedia encyclopedias. Overall, Wikipedia comprises more than 40 million articles in 301 different languages and by February 2014 it had reached 18 billion page views and nearly 500 million unique visitors per month.

In 2005, Nature published a peer review comparing 42 science articles from Encyclopædia Britannica and Wikipedia and found that Wikipedia's level of accuracy approached that of Britannica. Time magazine stated that the open-door policy of allowing anyone to edit had made Wikipedia the biggest and possibly the best encyclopedia in the world, and was a testament to the vision of Jimmy Wales.

Wikipedia has been criticized for exhibiting systemic bias, for presenting a mixture of "truths, half truths, and some falsehoods", and for being subject to manipulation and spin in controversial topics. In 2017, Facebook announced that it would help readers detect fake news by suitable links to Wikipedia articles. YouTube announced a similar plan in 2018.

History

Jimmy Wales and Larry Sanger

Nupedia

Wikipedia originally developed from another encyclopedia project called Nupedia

Other collaborative online encyclopedias were attempted before Wikipedia, but none were as successful. Wikipedia began as a complementary project for Nupedia, a free online English-language encyclopedia project whose articles were written by experts and reviewed under a formal process. It was founded on March 9, 2000, under the ownership of Bomis, a web portal company. Its main figures were Bomis CEO Jimmy Wales and Larry Sanger, editor-in-chief for Nupedia and later Wikipedia. Nupedia was initially licensed under its own Nupedia Open Content License, but even before Wikipedia was founded, Nupedia switched to the GNU Free Documentation License at the urging of Richard Stallman. Wales is credited with defining the goal of making a publicly editable encyclopedia, while Sanger is credited with the strategy of using a wiki to reach that goal. On January 10, 2001, Sanger proposed on the Nupedia mailing list to create a wiki as a "feeder" project for Nupedia.

Launch and early growth

The domains wikipedia.com and wikipedia.org were registered on January 12, 2001 and January 13, 2001 respectively, and Wikipedia was launched on January 15, 2001, as a single English-language edition at www.wikipedia.com, and announced by Sanger on the Nupedia mailing list. Wikipedia's policy of "neutral point-of-view" was codified in its first months. Otherwise, there were relatively few rules initially and Wikipedia operated independently of Nupedia. Originally, Bomis intended to make Wikipedia a business for profit.

Wikipedia gained early contributors from Nupedia, Slashdot postings, and web search engine indexing. Language editions were also created, with a total of 161 by the end of 2004. Nupedia and Wikipedia coexisted until the former's servers were taken down permanently in 2003, and its text was incorporated into Wikipedia. The English Wikipedia passed the mark of two million articles on September 9, 2007, making it the largest encyclopedia ever assembled, surpassing the 1408 Yongle Encyclopedia, which had held the record for almost 600 years.

Citing fears of commercial advertising and lack of control in Wikipedia, users of the Spanish Wikipedia forked from Wikipedia to create the Enciclopedia Libre in February 2002. These moves encouraged Wales to announce that Wikipedia would not display advertisements, and to change Wikipedia's domain from wikipedia.com to wikipedia.org.

Though the English Wikipedia reached three million articles in August 2009, the growth of the edition, in terms of the numbers of new articles and of contributors, appears to have peaked around early 2007. Around 1,800 articles were added daily to the encyclopedia in 2006; by 2013 that average was roughly 800. A team at the Palo Alto Research Center attributed this slowing of growth to the project's increasing exclusivity and resistance to change. Others suggest that the growth is flattening naturally because articles that could be called "low-hanging fruit"—topics that clearly merit an article—have already been created and built up extensively.

In November 2009, a researcher at the Rey Juan Carlos University in Madrid (Spain) found that the English Wikipedia had lost 49,000 editors during the first three months of 2009; in comparison, the project lost only 4,900 editors during the same period in 2008. The Wall Street Journal cited the array of rules applied to editing and disputes related to such content among the reasons for this trend. Wales disputed these claims in 2009, denying the decline and questioning the methodology of the study. Two years later, in 2011, Wales acknowledged the presence of a slight decline, noting a decrease from "a little more than 36,000 writers" in June 2010 to 35,800 in June 2011. In the same interview, Wales also claimed the number of editors was "stable and sustainable". A 2013 article titled "The Decline of Wikipedia" in MIT's Technology Review questioned this claim. The article revealed that since 2007, Wikipedia had lost a third of the volunteer editors who update and correct the online encyclopedia and those still there have focused increasingly on minutiae. In July 2012, The Atlantic reported that the number of administrators is also in decline. In the November 25, 2013, issue of New York magazine, Katherine Ward stated "Wikipedia, the sixth-most-used website, is facing an internal crisis".

Wikipedia blackout protest against SOPA on January 18, 2012

Milestones

In January 2007, Wikipedia entered for the first time the top-ten list of the most popular websites in the U.S., according to comScore Networks. With 42.9 million unique visitors, Wikipedia was ranked number 9, surpassing The New York Times (#10) and Apple (#11). This marked a significant increase over January 2006, when the rank was number 33, with Wikipedia receiving around 18.3 million unique visitors. As of March 2015, Wikipedia has rank 5 among websites in terms of popularity according to Alexa Internet. In 2014, it received 8 billion pageviews every month. On February 9, 2014, The New York Times reported that Wikipedia has 18 billion page views and nearly 500 million unique visitors a month, "according to the ratings firm comScore."

On January 18, 2012, the English Wikipedia participated in a series of coordinated protests against two proposed laws in the United States Congress—the Stop Online Piracy Act (SOPA) and the PROTECT IP Act (PIPA)—by blacking out its pages for 24 hours. More than 162 million people viewed the blackout explanation page that temporarily replaced Wikipedia content.

The Wikipedia Page on December 17, 2001

Loveland and Reagle argue that, in process, Wikipedia follows a long tradition of historical encyclopedias that accumulated improvements piecemeal through "stigmergic accumulation".

On January 20, 2014, Subodh Varma reporting for The Economic Times indicated that not only had Wikipedia's growth flattened, but that it "had lost nearly 10 per cent of its page views last year. There was a decline of about 2 billion between December 2012 and December 2013. Its most popular versions are leading the slide: page-views of the English Wikipedia declined by 12 per cent, those of German version slid by 17 per cent and the Japanese version lost 9 per cent." Varma added that, "While Wikipedia's managers think that this could be due to errors in counting, other experts feel that Google's Knowledge Graphs project launched last year may be gobbling up Wikipedia users." When contacted on this matter, Clay Shirky, associate professor at New York University and fellow at Harvard's Berkman Center for Internet and Security indicated that he suspected much of the page view decline was due to Knowledge Graphs, stating, "If you can get your question answered from the search page, you don't need to click [any further]."

By the end of December 2016, Wikipedia was ranked fifth in the most popular websites globally.

Openness

Number of English Wikipedia articles

Wikipedia editors with >100 edits per month

Differences between versions of an article are highlighted as shown

Unlike traditional encyclopedias, Wikipedia follows the procrastination principle regarding the security of its content. It started almost entirely open—anyone could create articles, and any Wikipedia article could be edited by any reader, even those who did not have a Wikipedia account. Modifications to all articles would be published immediately. As a result, any article could contain inaccuracies such as errors, ideological biases, and nonsensical or irrelevant text.

Restrictions

Due to the increasing popularity of Wikipedia, some editions, including the English version, have introduced editing restrictions in some cases. For instance, on the English Wikipedia and some other language editions, only registered users may create a new article. On the English Wikipedia, among others, some particularly controversial, sensitive and/or vandalism-prone pages have been protected to some degree. A frequently vandalized article can be semi-protected or extended confirmed protected, meaning that only autoconfirmed or extended confirmed editors are able to modify it. A particularly contentious article may be locked so that only administrators are able to make changes.

In certain cases, all editors are allowed to submit modifications, but review is required for some editors, depending on certain conditions. For example, the German Wikipedia maintains "stable versions" of articles, which have passed certain reviews. Following protracted trials and community discussion, the English Wikipedia introduced the "pending changes" system in December 2012. Under this system, new and unregistered users' edits to certain controversial or vandalism-prone articles are reviewed by established users before they are published.

The editing interface of Wikipedia

Review of changes

Although changes are not systematically reviewed, the software that powers Wikipedia provides certain tools allowing anyone to review changes made by others. The "History" page of each article links to each revision. On most articles, anyone can undo others' changes by clicking a link on the article's history page. Anyone can view the latest changes to articles, and anyone may maintain a "watchlist" of articles that interest them so they can be notified of any changes. "New pages patrol" is a process whereby newly created articles are checked for obvious problems.

In 2003, economics PhD student Andrea Ciffolilli argued that the low transaction costs of participating in a wiki create a catalyst for collaborative development, and that features such as allowing easy access to past versions of a page favor "creative construction" over "creative destruction".

Vandalism

Any change or edit that manipulates content in a way that purposefully compromises the integrity of Wikipedia is considered vandalism. The most common and obvious types of vandalism include additions of obscenities and crude humor. Vandalism can also include advertising and other types of spam. Sometimes editors commit vandalism by removing content or entirely blanking a given page. Less common types of vandalism, such as the deliberate addition of plausible but false information to an article, can be more difficult to detect. Vandals can introduce irrelevant formatting, modify page semantics such as the page's title or categorization, manipulate the underlying code of an article, or use images disruptively.

American journalist John Seigenthaler (1927–2014), subject of the Seigenthaler incident

Obvious vandalism is generally easy to remove from Wikipedia articles; the median time to detect and fix vandalism is a few minutes. However, some vandalism takes much longer to repair.

In the Seigenthaler biography incident, an anonymous editor introduced false information into the biography of American political figure John Seigenthaler in May 2005. Seigenthaler was falsely presented as a suspect in the assassination of John F. Kennedy. The article remained uncorrected for four months. Seigenthaler, the founding editorial director of USA Today and founder of the Freedom Forum First Amendment Center at Vanderbilt University, called Wikipedia co-founder Jimmy Wales and asked whether he had any way of knowing who contributed the misinformation. Wales replied that he did not, although the perpetrator was eventually traced. After the incident, Seigenthaler described Wikipedia as "a flawed and irresponsible research tool". This incident led to policy changes at Wikipedia, specifically targeted at tightening up the verifiability of biographical articles of living people.

Policies and laws

Content in Wikipedia is subject to the laws (in particular, copyright laws) of the United States and of the U.S. state of Virginia, where the majority of Wikipedia's servers are located. Beyond legal matters, the editorial principles of Wikipedia are embodied in the "five pillars" and in numerous policies and guidelines intended to appropriately shape content. Even these rules are stored in wiki form, and Wikipedia editors write and revise the website's policies and guidelines. Editors can enforce these rules by deleting or modifying non-compliant material. Originally, rules on the non-English editions of Wikipedia were based on a translation of the rules for the English Wikipedia. They have since diverged to some extent.

Content policies and guidelines

According to the rules on the English Wikipedia, each entry in Wikipedia must be about a topic that is encyclopedic and is not a dictionary entry or dictionary-like. A topic should also meet Wikipedia's standards of "notability", which generally means that the topic must have been covered in mainstream media or major academic journal sources that are independent of the article's subject. Further, Wikipedia intends to convey only knowledge that is already established and recognized. It must not present original research. A claim that is likely to be challenged requires a reference to a reliable source. Among Wikipedia editors, this is often phrased as "verifiability, not truth" to express the idea that the readers, not the encyclopedia, are ultimately responsible for checking the truthfulness of the articles and making their own interpretations. This can at times lead to the removal of information that, though valid, is not properly sourced. Finally, Wikipedia must not take sides. All opinions and viewpoints, if attributable to external sources, must enjoy an appropriate share of coverage within an article. This is known as neutral point of view (NPOV).

Governance

Wikipedia's initial anarchy integrated democratic and hierarchical elements over time. An article is not considered to be owned by its creator or any other editor, nor by the subject of the article. Wikipedia's contributors avoid a tragedy of the commons (behaving contrary to the common good) by internalizing benefits. They do this by experiencing flow (i.e., energized focus, full involvement, and enjoyment) and identifying with and gaining status in the Wikipedia community.

Administrators

Editors in good standing in the community can run for one of many levels of volunteer stewardship: this begins with "administrator", privileged users who can delete pages, prevent articles from being changed in case of vandalism or editorial disputes (setting protective measures on articles), and try to prevent certain persons from editing. Despite the name, administrators are not supposed to enjoy any special privilege in decision-making; instead, their powers are mostly limited to making edits that have project-wide effects and thus are disallowed to ordinary editors, and to implement restrictions intended to prevent certain persons from making disruptive edits (such as vandalism).

Fewer editors become administrators than in years past, in part because the process of vetting potential Wikipedia administrators has become more rigorous.

Bureaucrats name new administrators, solely upon the recommendations from the community.

Dispute resolution

Wikipedians often have disputes regarding content, which may result in repeatedly making opposite changes to an article, known as edit warring. Over time, Wikipedia has developed a semi-formal dispute resolution process to assist in such circumstances. In order to determine community consensus, editors can raise issues at appropriate community forums, or seek outside input through third opinion requests or by initiating a more general community discussion known as a request for comment.

Arbitration Committee

The Arbitration Committee presides over the ultimate dispute resolution process. Although disputes usually arise from a disagreement between two opposing views on how an article should read, the Arbitration Committee explicitly refuses to directly rule on the specific view that should be adopted. Statistical analyses suggest that the committee ignores the content of disputes and rather focuses on the way disputes are conducted, functioning not so much to resolve disputes and make peace between conflicting editors, but to weed out problematic editors while allowing potentially productive editors back in to participate. Therefore, the committee does not dictate the content of articles, although it sometimes condemns content changes when it deems the new content violates Wikipedia policies (for example, if the new content is considered biased). Its remedies include cautions and probations (used in 63% of cases) and banning editors from articles (43%), subject matters (23%), or Wikipedia (16%). Complete bans from Wikipedia are generally limited to instances of impersonation and anti-social behavior. When conduct is not impersonation or anti-social, but rather anti-consensus or in violation of editing policies, remedies tend to be limited to warnings.

Community

Wikimania 2005 – an annual conference for users of Wikipedia and other projects operated by the Wikimedia Foundation, was held in Frankfurt am Main, Germany from August 4 to 8.

Each article and each user of Wikipedia has an associated "Talk" page. These form the primary communication channel for editors to discuss, coordinate and debate.

Wikipedians and British Museum curators collaborate on the article Hoxne Hoard in June 2010

Wikipedia's community has been described as cult-like, although not always with entirely negative connotations. The project's preference for cohesiveness, even if it requires compromise that includes disregard of credentials, has been referred to as "anti-elitism".

Wikipedians sometimes award one another virtual barnstars for good work. These personalized tokens of appreciation reveal a wide range of valued work extending far beyond simple editing to include social support, administrative actions, and types of articulation work.

Wikipedia does not require that its editors and contributors provide identification. As Wikipedia grew, "Who writes Wikipedia?" became one of the questions frequently asked on the project. Jimmy Wales once argued that only "a community ... a dedicated group of a few hundred volunteers" makes the bulk of contributions to Wikipedia and that the project is therefore "much like any traditional organization". In 2008, a Slate magazine article reported that: "According to researchers in Palo Alto, 1 percent of Wikipedia users are responsible for about half of the site's edits." This method of evaluating contributions was later disputed by Aaron Swartz, who noted that several articles he sampled had large portions of their content (measured by number of characters) contributed by users with low edit counts.

The English Wikipedia has 5,790,638 articles, 35,473,236 registered editors, and 125,285 active editors. An editor is considered active if they have made one or more edits in the past thirty days. 

Editors who fail to comply with Wikipedia cultural rituals, such as signing talk page comments, may implicitly signal that they are Wikipedia outsiders, increasing the odds that Wikipedia insiders may target or discount their contributions. Becoming a Wikipedia insider involves non-trivial costs: the contributor is expected to learn Wikipedia-specific technological codes, submit to a sometimes convoluted dispute resolution process, and learn a "baffling culture rich with in-jokes and insider references". Editors who do not log in are in some sense second-class citizens on Wikipedia, as "participants are accredited by members of the wiki community, who have a vested interest in preserving the quality of the work product, on the basis of their ongoing participation", but the contribution histories of anonymous unregistered editors recognized only by their IP addresses cannot be attributed to a particular editor with certainty.

Studies

A 2007 study by researchers from Dartmouth College found that "anonymous and infrequent contributors to Wikipedia [...] are as reliable a source of knowledge as those contributors who register with the site". Jimmy Wales stated in 2009 that "(I)t turns out over 50% of all the edits are done by just .7% of the users... 524 people... And in fact the most active 2%, which is 1400 people, have done 73.4% of all the edits." However, Business Insider editor and journalist Henry Blodget showed in 2009 that in a random sample of articles, most content in Wikipedia (measured by the amount of contributed text that survives to the latest sampled edit) is created by "outsiders", while most editing and formatting is done by "insiders".

A 2008 study found that Wikipedians were less agreeable, open, and conscientious than others, although a later commentary pointed out serious flaws, including that the data showed higher openness and that the differences with the control group and the samples were small. According to a 2009 study, there is "evidence of growing resistance from the Wikipedia community to new content".

Diversity

Several studies have shown that most of the Wikipedia contributors are male. Notably, the results of a Wikimedia Foundation survey in 2008 showed that only 13% of Wikipedia editors were female. Because of this, universities throughout the United States tried to encourage females to become Wikipedia contributors. Similarly, many of these universities, including Yale and Brown, gave college credit to students who create or edit an article relating to women in science or technology. Andrew Lih, a professor and scientist, wrote in The New York Times that the reason he thought the number of male contributors outnumbered the number of females so greatly, is because identifying as a feminist may expose oneself to "ugly, intimidating behavior." Data has shown that Africans are underrepresented among Wikipedia editors.

Language editions

There are currently 301 language editions of Wikipedia (also called language versions, or simply Wikipedias). Fifteen of these have over one million articles each (English, Cebuano, Swedish, German, Dutch, French, Russian, Italian, Spanish, Waray-Waray, Polish, Vietnamese, Japanese, Chinese and Portuguese), four more have over 500,000 articles (Ukrainian, Persian, Catalan and Arabic), 40 more have over 100,000 articles, and 78 more have over 10,000 articles. The largest, the English Wikipedia, has over 5.7 million articles. As of January 2019, according to Alexa, the English subdomain (en.wikipedia.org; English Wikipedia) receives approximately 57% of Wikipedia's cumulative traffic, with the remaining split among the other languages (Russian: 9%; Chinese: 6%; Japanese: 6%; Spanish: 5%). As of January 2019, the six largest language editions are (in order of article count) the English, Cebuano, Swedish, German, French, and Dutch Wikipedias.

A graph for pageviews of Turkish Wikipedia shows a great drop of roughly 80 % immediately after the block of Wikipedia in Turkey was imposed in 2017.

Since Wikipedia is based on the Web and therefore worldwide, contributors to the same language edition may use different dialects or may come from different countries (as is the case for the English edition). These differences may lead to some conflicts over spelling differences (e.g. colour versus color) or points of view.

Though the various language editions are held to global policies such as "neutral point of view", they diverge on some points of policy and practice, most notably on whether images that are not licensed freely may be used under a claim of fair use.

Jimmy Wales has described Wikipedia as "an effort to create and distribute a free encyclopedia of the highest possible quality to every single person on the planet in their own language". Though each language edition functions more or less independently, some efforts are made to supervise them all. They are coordinated in part by Meta-Wiki, the Wikimedia Foundation's wiki devoted to maintaining all of its projects (Wikipedia and others). For instance, Meta-Wiki provides important statistics on all language editions of Wikipedia, and it maintains a list of articles every Wikipedia should have. The list concerns basic content by subject: biography, history, geography, society, culture, science, technology, and mathematics. It is not rare for articles strongly related to a particular language not to have counterparts in another edition. For example, articles about small towns in the United States might only be available in English, even when they meet notability criteria of other language Wikipedia projects. 

Estimation of contributions shares from different regions in the world to different Wikipedia editions

Translated articles represent only a small portion of articles in most editions, in part because those edition do not allow fully automated translation of articles. Articles available in more than one language may offer "interwiki links", which link to the counterpart articles in other editions.

A study published by PLoS ONE in 2012 also estimated the share of contributions to different editions of Wikipedia from different regions of the world. It reported that the proportion of the edits made from North America was 51% for the English Wikipedia, and 25% for the simple English Wikipedia. The Wikimedia Foundation hopes to increase the number of editors in the Global South to 37% by 2015.

English Wikipedia editor decline

On March 1, 2014, The Economist in an article titled "The Future of Wikipedia" cited a trend analysis concerning data published by Wikimedia stating that: "The number of editors for the English-language version has fallen by a third in seven years." The attrition rate for active editors in English Wikipedia was cited by The Economist as substantially in contrast to statistics for Wikipedia in other languages (non-English Wikipedia). The Economist reported that the number of contributors with an average of five of more edits per month was relatively constant since 2008 for Wikipedia in other languages at approximately 42,000 editors within narrow seasonal variances of about 2,000 editors up or down. The attrition rates for editors in English Wikipedia, by sharp comparison, were cited as peaking in 2007 at approximately 50,000 editors, which has dropped to 30,000 editors as of the start of 2014. 

At the quoted trend rate, the number of active editors in English Wikipedia has lost approximately 20,000 editors to attrition since 2007, and the documented trend rate indicates the loss of another 20,000 editors by 2021, down to 10,000 active editors on English Wikipedia by 2021 if left unabated. Given that the trend analysis published in The Economist presents the number of active editors for Wikipedia in other languages (non-English Wikipedia) as remaining relatively constant and successful in sustaining its numbers at approximately 42,000 active editors, the contrast has pointed to the effectiveness of Wikipedia in other languages to retain its active editors on a renewable and sustained basis. No comment was made concerning which of the differentiated edit policy standards from Wikipedia in other languages (non-English Wikipedia) would provide a possible alternative to English Wikipedia for effectively ameliorating substantial editor attrition rates on the English-language Wikipedia.

Reception

Several Wikipedians have criticized Wikipedia's large and growing regulation, which includes over 50 policies and nearly 150,000 words as of 2014.

Critics have stated that Wikipedia exhibits systemic bias. In 2010, columnist and journalist Edwin Black criticized Wikipedia for being a mixture of "truth, half truth, and some falsehoods". Articles in The Chronicle of Higher Education and The Journal of Academic Librarianship have criticized Wikipedia's Undue Weight policy, concluding that the fact that Wikipedia explicitly is not designed to provide correct information about a subject, but rather focus on all the major viewpoints on the subject and give less attention to minor ones, creates omissions that can lead to false beliefs based on incomplete information.

Journalists Oliver Kamm and Edwin Black noted (in 2010 and 2011 respectively) how articles are dominated by the loudest and most persistent voices, usually by a group with an "ax to grind" on the topic. A 2008 article in Education Next Journal concluded that as a resource about controversial topics, Wikipedia is subject to manipulation and spin.

In 2006, the Wikipedia Watch criticism website listed dozens of examples of plagiarism in the English Wikipedia.

Accuracy of content

Articles for traditional encyclopedias such as Encyclopædia Britannica are carefully and deliberately written by experts, lending such encyclopedias a reputation for accuracy. However, a peer review in 2005 of forty-two scientific entries on both Wikipedia and Encyclopædia Britannica by the science journal Nature found few differences in accuracy, and concluded that "the average science entry in Wikipedia contained around four inaccuracies; Britannica, about three." Reagle suggested that while the study reflects "a topical strength of Wikipedia contributors" in science articles, "Wikipedia may not have fared so well using a random sampling of articles or on humanities subjects." The findings by Nature were disputed by Encyclopædia Britannica, and in response, Nature gave a rebuttal of the points raised by Britannica. In addition to the point-for-point disagreement between these two parties, others have examined the sample size and selection method used in the Nature effort, and suggested a "flawed study design" (in Nature's manual selection of articles, in part or in whole, for comparison), absence of statistical analysis (e.g., of reported confidence intervals), and a lack of study "statistical power" (i.e., owing to small sample size, 42 or 4 × 10¹ articles compared, vs greater than 10⁵ and greater than 10⁶ set sizes for Britannica and the English Wikipedia, respectively).

As a consequence of the open structure, Wikipedia "makes no guarantee of validity" of its content, since no one is ultimately responsible for any claims appearing in it. Concerns have been raised by PC World in 2009 regarding the lack of accountability that results from users' anonymity, the insertion of false information, vandalism, and similar problems.

Economist Tyler Cowen wrote: "If I had to guess whether Wikipedia or the median refereed journal article on economics was more likely to be true, after a not so long think I would opt for Wikipedia." He comments that some traditional sources of non-fiction suffer from systemic biases and novel results, in his opinion, are over-reported in journal articles and relevant information is omitted from news reports. However, he also cautions that errors are frequently found on Internet sites, and that academics and experts must be vigilant in correcting them.

Critics argue that Wikipedia's open nature and a lack of proper sources for most of the information makes it unreliable. Some commentators suggest that Wikipedia may be reliable, but that the reliability of any given article is not clear. Editors of traditional reference works such as the Encyclopædia Britannica have questioned the project's utility and status as an encyclopedia. Wikipedia co-founder Jimmy Wales has claimed that Wikipedia has largely avoided the problem of "fake news" because the Wikipedia community regularly debates the quality of sources in articles.

Wikipedia's open structure inherently makes it an easy target for Internet trolls, spammers, and various forms of paid advocacy seen as counterproductive to the maintenance of a neutral and verifiable online encyclopedia. In response to paid advocacy editing and undisclosed editing issues, Wikipedia was reported in an article in The Wall Street Journal, to have strengthened its rules and laws against undisclosed editing. The article stated that: "Beginning Monday [from the date of article, June 16, 2014], changes in Wikipedia's terms of use will require anyone paid to edit articles to disclose that arrangement. Katherine Maher, the nonprofit Wikimedia Foundation's chief communications officer, said the changes address a sentiment among volunteer editors that, 'we're not an advertising service; we're an encyclopedia.'" These issues, among others, had been parodied since the first decade of Wikipedia, notably by Stephen Colbert on The Colbert Report.

A Harvard law textbook, Legal Research in a Nutshell (2011), cites Wikipedia as a "general source" that "can be a real boon" in "coming up to speed in the law governing a situation" and, "while not authoritative, can provide basic facts as well as leads to more in-depth resources".

Discouragement in education

Most university lecturers discourage students from citing any encyclopedia in academic work, preferring primary sources; some specifically prohibit Wikipedia citations. Wales stresses that encyclopedias of any type are not usually appropriate to use as citable sources, and should not be relied upon as authoritative. Wales once (2006 or earlier) said he receives about ten emails weekly from students saying they got failing grades on papers because they cited Wikipedia; he told the students they got what they deserved. "For God's sake, you're in college; don't cite the encyclopedia", he said.

In February 2007, an article in The Harvard Crimson newspaper reported that a few of the professors at Harvard University were including Wikipedia articles in their syllabi, although without realizing the articles might change. In June 2007, former president of the American Library Association Michael Gorman condemned Wikipedia, along with Google, stating that academics who endorse the use of Wikipedia are "the intellectual equivalent of a dietitian who recommends a steady diet of Big Macs with everything".

Medical information

On March 5, 2014, Julie Beck writing for The Atlantic magazine in an article titled "Doctors' #1 Source for Healthcare Information: Wikipedia", stated that "Fifty percent of physicians look up conditions on the (Wikipedia) site, and some are editing articles themselves to improve the quality of available information." Beck continued to detail in this article new programs of Amin Azzam at the University of San Francisco to offer medical school courses to medical students for learning to edit and improve Wikipedia articles on health-related issues, as well as internal quality control programs within Wikipedia organized by James Heilman to improve a group of 200 health-related articles of central medical importance up to Wikipedia's highest standard of articles using its Featured Article and Good Article peer review evaluation process. In a May 7, 2014, follow-up article in The Atlantic titled "Can Wikipedia Ever Be a Definitive Medical Text?", Julie Beck quotes WikiProject Medicine's James Heilman as stating: "Just because a reference is peer-reviewed doesn't mean it's a high-quality reference." Beck added that: "Wikipedia has its own peer review process before articles can be classified as 'good' or 'featured.' Heilman, who has participated in that process before, says 'less than 1 percent' of Wikipedia's medical articles have passed."

Quality of writing

In 2008, researchers at Carnegie Mellon University found that the quality of a Wikipedia article would suffer rather than gain from adding more writers when the article lacked appropriate explicit or implicit coordination. For instance, when contributors rewrite small portions of an entry rather than making full-length revisions, high- and low-quality content may be intermingled within an entry. Roy Rosenzweig, a history professor, stated that American National Biography Online outperformed Wikipedia in terms of its "clear and engaging prose", which, he said, was an important aspect of good historical writing. Contrasting Wikipedia's treatment of Abraham Lincoln to that of Civil War historian James McPherson in American National Biography Online, he said that both were essentially accurate and covered the major episodes in Lincoln's life, but praised "McPherson's richer contextualization [...] his artful use of quotations to capture Lincoln's voice [...] and [...] his ability to convey a profound message in a handful of words." By contrast, he gives an example of Wikipedia's prose that he finds "both verbose and dull". Rosenzweig also criticized the "waffling—encouraged by the NPOV policy—[which] means that it is hard to discern any overall interpretive stance in Wikipedia history". While generally praising the article on William Clarke Quantrill, he quoted its conclusion as an example of such "waffling", which then stated: "Some historians [...] remember him as an opportunistic, bloodthirsty outlaw, while others continue to view him as a daring soldier and local folk hero."

Other critics have made similar charges that, even if Wikipedia articles are factually accurate, they are often written in a poor, almost unreadable style. Frequent Wikipedia critic Andrew Orlowski commented, "Even when a Wikipedia entry is 100 per cent factually correct, and those facts have been carefully chosen, it all too often reads as if it has been translated from one language to another then into a third, passing an illiterate translator at each stage." A study of Wikipedia articles on cancer was conducted in 2010 by Yaacov Lawrence of the Kimmel Cancer Center at Thomas Jefferson University. The study was limited to those articles that could be found in the Physician Data Query and excluded those written at the "start" class or "stub" class level. Lawrence found the articles accurate but not very readable, and thought that "Wikipedia's lack of readability (to non-college readers) may reflect its varied origins and haphazard editing". The Economist argued that better-written articles tend to be more reliable: "inelegant or ranting prose usually reflects muddled thoughts and incomplete information".

To assess Wikipedia articles it can be used various quality measures related to credibility, completeness, objectivity, readability, relevance, style and timeliness.

Coverage of topics and systemic bias

Wikipedia seeks to create a summary of all human knowledge in the form of an online encyclopedia, with each topic covered encyclopedically in one article. Since it has terabytes of disk space, it can have far more topics than can be covered by any printed encyclopedia. The exact degree and manner of coverage on Wikipedia is under constant review by its editors, and disagreements are not uncommon. Wikipedia contains materials that some people may find objectionable, offensive, or pornographic because Wikipedia is not censored. The policy has sometimes proved controversial: in 2008, Wikipedia rejected an online petition against the inclusion of images of Muhammad in the English edition of its Muhammad article, citing this policy. The presence of politically, religiously, and pornographically sensitive materials in Wikipedia has led to the censorship of Wikipedia by national authorities in China, and Pakistan, among other countries. 

Pie chart of Wikipedia content by subject as of January 2008

A 2008 study conducted by researchers at Carnegie Mellon University and Palo Alto Research Center gave a distribution of topics as well as growth (from July 2006 to January 2008) in each field:

• Culture and the arts: 30% (210%)
• Biographies and persons: 15% (97%)
• Geography and places: 14% (52%)
• Society and social sciences: 12% (83%)
• History and events: 11% (143%)
• Natural and physical sciences: 9% (213%)
• Technology and the applied sciences: 4% (−6%)
• Religions and belief systems: 2% (38%)
• Health: 2% (42%)
• Mathematics and logic: 1% (146%)
• Thought and philosophy: 1% (160%)

These numbers refer only to the quantity of articles: it is possible for one topic to contain a large number of short articles and another to contain a small number of large ones. Through its "Wikipedia Loves Libraries" program, Wikipedia has partnered with major public libraries such as the New York Public Library for the Performing Arts to expand its coverage of underrepresented subjects and articles.

A 2011 study conducted by researchers at the University of Minnesota indicated that male and female editors focus on different coverage topics. There was a greater concentration of females in the People and Arts category, while males focus more on Geography and Science.

Coverage of topics and selection bias

Research conducted by Mark Graham of the Oxford Internet Institute in 2009 indicated that the geographic distribution of article topics is highly uneven. Africa is most underrepresented. Across 30 language editions of Wikipedia, historical articles and sections are generally Eurocentric and focused on recent events.

An editorial in The Guardian in 2014 noted that women porn stars are better covered than women writers as a further example. Data has also shown that Africa-related material often faces omission; a knowledge gap that a July 2018 Wikimedia conference in Cape Town sought to address.

Systemic bias

When multiple editors contribute to one topic or set of topics, systemic bias may arise, due to the demographic backgrounds of the editors. In 2011, Wales noted that the unevenness of coverage is a reflection of the demography of the editors, which predominantly consists of highly educated young males in the developed world (see previously). The October 22, 2013, essay by Tom Simonite in MIT's Technology Review titled "The Decline of Wikipedia" discussed the effect of systemic bias and policy creep on the downward trend in the number of editors.

Systemic bias on Wikipedia may follow that of culture generally, for example favoring certain nationalities, ethnicities or majority religions. It may more specifically follow the biases of Internet culture, inclining to being young, male, English-speaking, educated, technologically aware, and wealthy enough to spare time for editing. Biases of its own may include over-emphasis on topics such as pop culture, technology, and current events.

Taha Yasseri of the University of Oxford, in 2013, studied the statistical trends of systemic bias at Wikipedia introduced by editing conflicts and their resolution. His research examined the counterproductive work behavior of edit warring. Yasseri contended that simple reverts or "undo" operations were not the most significant measure of counterproductive behavior at Wikipedia and relied instead on the statistical measurement of detecting "reverting/reverted pairs" or "mutually reverting edit pairs". Such a "mutually reverting edit pair" is defined where one editor reverts the edit of another editor who then, in sequence, returns to revert the first editor in the "mutually reverting edit pairs". The results were tabulated for several language versions of Wikipedia. The English Wikipedia's three largest conflict rates belonged to the articles George W. Bush, Anarchism and Muhammad. By comparison, for the German Wikipedia, the three largest conflict rates at the time of the Oxford study were for the articles covering (i) Croatia, (ii) Scientology and (iii) 9/11 conspiracy theories.

Researchers from the Washington University developed a statistical model to measure systematic bias in the behavior of Wikipedia's users regarding controversial topics. The authors focused on behavioral changes of the encyclopedia's administrators after assuming the post, writing that systematic bias occurred after the fact.

Explicit content

Wikipedia has been criticized for allowing information of graphic content. Articles depicting what some critics have called objectionable content (such as Feces, Cadaver, Human penis, Vulva, and Nudity) contain graphic pictures and detailed information easily available to anyone with access to the internet, including children.

The site also includes sexual content such as images and videos of masturbation and ejaculation, illustrations of zoophilia, and photos from hardcore pornographic films in its articles. It also has non-sexual photographs of nude children. 

The Wikipedia article about Virgin Killer—a 1976 album from German heavy metal band Scorpions—features a picture of the album's original cover, which depicts a naked prepubescent girl. The original release cover caused controversy and was replaced in some countries. In December 2008, access to the Wikipedia article Virgin Killer was blocked for four days by most Internet service providers in the United Kingdom after the Internet Watch Foundation (IWF) decided the album cover was a potentially illegal indecent image and added the article's URL to a "blacklist" it supplies to British internet service providers.

In April 2010, Sanger wrote a letter to the Federal Bureau of Investigation, outlining his concerns that two categories of images on Wikimedia Commons contained child pornography, and were in violation of US federal obscenity law. Sanger later clarified that the images, which were related to pedophilia and one about lolicon, were not of real children, but said that they constituted "obscene visual representations of the sexual abuse of children", under the PROTECT Act of 2003. That law bans photographic child pornography and cartoon images and drawings of children that are obscene under American law. Sanger also expressed concerns about access to the images on Wikipedia in schools. Wikimedia Foundation spokesman Jay Walsh strongly rejected Sanger's accusation, saying that Wikipedia did not have "material we would deem to be illegal. If we did, we would remove it." Following the complaint by Sanger, Wales deleted sexual images without consulting the community. After some editors who volunteer to maintain the site argued that the decision to delete had been made hastily, Wales voluntarily gave up some of the powers he had held up to that time as part of his co-founder status. He wrote in a message to the Wikimedia Foundation mailing-list that this action was "in the interest of encouraging this discussion to be about real philosophical/content issues, rather than be about me and how quickly I acted". Critics, including Wikipediocracy, noticed that many of the pornographic images deleted from Wikipedia since 2010 have reappeared.

Privacy

One privacy concern in the case of Wikipedia is the right of a private citizen to remain a "private citizen" rather than a "public figure" in the eyes of the law. It is a battle between the right to be anonymous in cyberspace and the right to be anonymous in real life ("meatspace"). A particular problem occurs in the case of an individual who is relatively unimportant and for whom there exists a Wikipedia page against her or his wishes. 

In January 2006, a German court ordered the German Wikipedia shut down within Germany because it stated the full name of Boris Floricic, aka "Tron", a deceased hacker. On February 9, 2006, the injunction against Wikimedia Deutschland was overturned, with the court rejecting the notion that Tron's right to privacy or that of his parents was being violated.

Wikipedia has a "Volunteer Response Team" that uses the OTRS system to handle queries without having to reveal the identities of the involved parties. This is used, for example, in confirming the permission for using individual images and other media in the project.

Sexism

Wikipedia has been described as harboring a battleground culture of sexism and harassment. The perceived toxic attitudes and tolerance of violent and abusive language are also reasons put forth for the gender gap in Wikipedia editors. In 2014, a female editor who requested a separate space on Wikipedia to discuss improving civility had her proposal referred to by a male editor using the words "the easiest way to avoid being called a cunt is not to act like one".

Operation

Wikimedia Foundation and Wikimedia movement affiliates

Katherine Maher is the third executive director at Wikimedia, following the departure of Lila Tretikov in 2016.

Wikipedia is hosted and funded by the Wikimedia Foundation, a non-profit organization which also operates Wikipedia-related projects such as Wiktionary and Wikibooks. The foundation relies on public contributions and grants to fund its mission. The foundation's 2013 IRS Form 990 shows revenue of $39.7 million and expenses of almost $29 million, with assets of $37.2 million and liabilities of about $2.3 million.

In May 2014, Wikimedia Foundation named Lila Tretikov as its second executive director, taking over for Sue Gardner. The Wall Street Journal reported on May 1, 2014, that Tretikov's information technology background from her years at University of California offers Wikipedia an opportunity to develop in more concentrated directions guided by her often repeated position statement that, "Information, like air, wants to be free." The same Wall Street Journal article reported these directions of development according to an interview with spokesman Jay Walsh of Wikimedia, who "said Tretikov would address that issue (paid advocacy) as a priority. 'We are really pushing toward more transparency... We are reinforcing that paid advocacy is not welcome.' Initiatives to involve greater diversity of contributors, better mobile support of Wikipedia, new geo-location tools to find local content more easily, and more tools for users in the second and third world are also priorities, Walsh said."

Following the departure of Tretikov from Wikipedia due to issues concerning the use of the "superprotection" feature which some language versions of Wikipedia have adopted, Katherine Maher became the third executive director the Wikimedia Foundation in June 2016. Maher has stated that one of her priorities would be the issue of editor harassment endemic to Wikipedia as identified by the Wikipedia board in December. Maher stated regarding the harassment issue that: "It establishes a sense within the community that this is a priority... (and that correction requires that) it has to be more than words."

Wikipedia is also supported by many organizations and groups that are affiliated with the Wikimedia Foundation but independently-run, called Wikimedia movement affiliates. These include Wikimedia chapters (which are national or sub-national organizations, such as Wikimedia Deutschland and Wikimédia France), thematic organizations (such as Amical Wikimedia for the Catalan language community), and user groups. These affiliates participate in the promotion, development, and funding of Wikipedia.

Software operations and support

The operation of Wikipedia depends on MediaWiki, a custom-made, free and open source wiki software platform written in PHP and built upon the MySQL database system. The software incorporates programming features such as a macro language, variables, a transclusion system for templates, and URL redirection. MediaWiki is licensed under the GNU General Public License and it is used by all Wikimedia projects, as well as many other wiki projects. Originally, Wikipedia ran on UseModWiki written in Perl by Clifford Adams (Phase I), which initially required CamelCase for article hyperlinks; the present double bracket style was incorporated later. Starting in January 2002 (Phase II), Wikipedia began running on a PHP wiki engine with a MySQL database; this software was custom-made for Wikipedia by Magnus Manske. The Phase II software was repeatedly modified to accommodate the exponentially increasing demand. In July 2002 (Phase III), Wikipedia shifted to the third-generation software, MediaWiki, originally written by Lee Daniel Crocker.

Several MediaWiki extensions are installed to extend the functionality of the MediaWiki software.

In April 2005, a Lucene extension was added to MediaWiki's built-in search and Wikipedia switched from MySQL to Lucene for searching. The site currently uses Lucene Search 2.1, which is written in Java and based on Lucene library 2.3.

In July 2013, after extensive beta testing, a WYSIWYG (What You See Is What You Get) extension, VisualEditor, was opened to public use. It was met with much rejection and criticism, and was described as "slow and buggy". The feature was changed from opt-out to opt-in afterward.

Automated editing

Computer programs called bots have been used widely to perform simple and repetitive tasks, such as correcting common misspellings and stylistic issues, or to start articles such as geography entries in a standard format from statistical data. One controversial contributor massively creating articles with his bot was reported to create up to ten thousand articles on the Swedish Wikipedia on certain days. There are also some bots designed to automatically notify editors when they make common editing errors (such as unmatched quotes or unmatched parentheses). Edits misidentified by a bot as the work of a banned editor can be restored by other editors. An anti-vandal bot tries to detect and revert vandalism quickly and automatically. Bots can also report edits from particular accounts or IP address ranges, as was done at the time of the MH17 jet downing incident in July 2014. Bots on Wikipedia must be approved prior to activation.

According to Andrew Lih, the current expansion of Wikipedia to millions of articles would be difficult to envision without the use of such bots.

Hardware operations and support

Wikipedia receives between 25,000 and 60,000 page requests per second, depending on time of day. As of 2008, page requests are first passed to a front-end layer of Squid caching servers. Further statistics, based on a publicly available 3-month Wikipedia access trace, are available. Requests that cannot be served from the Squid cache are sent to load-balancing servers running the Linux Virtual Server software, which in turn pass them to one of the Apache web servers for page rendering from the database. The web servers deliver pages as requested, performing page rendering for all the language editions of Wikipedia. To increase speed further, rendered pages are cached in a distributed memory cache until invalidated, allowing page rendering to be skipped entirely for most common page accesses. 

Overview of system architecture as of December 2010

Wikipedia currently runs on dedicated clusters of Linux servers (mainly Ubuntu). As of December 2009, there were 300 in Florida and 44 in Amsterdam. By January 22, 2013, Wikipedia had migrated its primary data center to an Equinix facility in Ashburn, Virginia.

Internal research and operational development

In accordance with growing amounts of incoming donations exceeding seven digits in 2013 as recently reported, the Foundation has reached a threshold of assets which qualify its consideration under the principles of industrial organization economics to indicate the need for the re-investment of donations into the internal research and development of the Foundation. Two of the recent projects of such internal research and development have been the creation of a Visual Editor and a largely under-utilized "Thank" tab which were developed for the purpose of ameliorating issues of editor attrition, which have met with limited success. The estimates for reinvestment by industrial organizations into internal research and development was studied by Adam Jaffe, who recorded that the range of 4% to 25% annually was to be recommended, with high end technology requiring the higher level of support for internal reinvestment. At the 2013 level of contributions for Wikimedia presently documented as 45 million dollars, the computed budget level recommended by Jaffe and Caballero for reinvestment into internal research and development is between 1.8 million and 11.3 million dollars annually. In 2016, the level of contributions were reported by Bloomberg News as being at $77 million annually, updating the Jaffe estimates for the higher level of support to between $3.08 million and $19.2 million annually.

Internal news publications

Community-produced news publications include the English Wikipedia's The Signpost, founded in 2005 by Michael Snow, an attorney, Wikipedia administrator and former chair of the Wikimedia Foundation board of trustees. It covers news and events from the site, as well as major events from other Wikimedia projects, such as Wikimedia Commons. Similar publications are the German-language Kurier, and the Portuguese-language Correio da Wikipédia. Other past and present community news publications on English Wikipedia include the "Wikiworld" web comic, the Wikipedia Weekly podcast, and newsletters of specific WikiProjects like The Bugle from WikiProject Military History and the monthly newsletter from The Guild of Copy Editors. There are also a number of publications from the Wikimedia Foundation and multilingual publications such as the Wikimedia Blog and This Month in Education.

Access to content

Content licensing

When the project was started in 2001, all text in Wikipedia was covered by the GNU Free Documentation License (GFDL), a copyleft license permitting the redistribution, creation of derivative works, and commercial use of content while authors retain copyright of their work. The GFDL was created for software manuals that come with free software programs licensed under the GPL. This made it a poor choice for a general reference work: for example, the GFDL requires the reprints of materials from Wikipedia to come with a full copy of the GFDL text. In December 2002, the Creative Commons license was released: it was specifically designed for creative works in general, not just for software manuals. The license gained popularity among bloggers and others distributing creative works on the Web. The Wikipedia project sought the switch to the Creative Commons. Because the two licenses, GFDL and Creative Commons, were incompatible, in November 2008, following the request of the project, the Free Software Foundation (FSF) released a new version of the GFDL designed specifically to allow Wikipedia to relicense its content to CC BY-SA by August 1, 2009. (A new version of the GFDL automatically covers Wikipedia contents.) In April 2009, Wikipedia and its sister projects held a community-wide referendum which decided the switch in June 2009.

The handling of media files (e.g. image files) varies across language editions. Some language editions, such as the English Wikipedia, include non-free image files under fair use doctrine, while the others have opted not to, in part because of the lack of fair use doctrines in their home countries (e.g. in Japanese copyright law). Media files covered by free content licenses (e.g. Creative Commons' CC BY-SA) are shared across language editions via Wikimedia Commons repository, a project operated by the Wikimedia Foundation. Wikipedia's accommodation of varying international copyright laws regarding images has led some to observe that its photographic coverage of topics lags behind the quality of the encyclopedic text.

The Wikimedia Foundation is not a licensor of content, but merely a hosting service for the contributors (and licensors) of the Wikipedia. This position has been successfully defended in court.

Methods of access

Because Wikipedia content is distributed under an open license, anyone can reuse or re-distribute it at no charge. The content of Wikipedia has been published in many forms, both online and offline, outside of the Wikipedia website.

• Websites – Thousands of "mirror sites" exist that republish content from Wikipedia: two prominent ones, that also include content from other reference sources, are Reference.com and Answers.com. Another example is Wapedia, which began to display Wikipedia content in a mobile-device-friendly format before Wikipedia itself did.
• Mobile apps – A variety of mobile apps provide access to Wikipedia on hand-held devices, including both Android and iOS devices.
• Search engines – Some web search engines make special use of Wikipedia content when displaying search results: examples include Bing (via technology gained from Powerset) and DuckDuckGo.
• Compact discs, DVDs – Collections of Wikipedia articles have been published on optical discs. An English version, 2006 Wikipedia CD Selection, contained about 2,000 articles. The Polish-language version contains nearly 240,000 articles. There are German- and Spanish-language versions as well. Also, "Wikipedia for Schools", the Wikipedia series of CDs / DVDs produced by Wikipedians and SOS Children, is a free, hand-checked, non-commercial selection from Wikipedia targeted around the UK National Curriculum and intended to be useful for much of the English-speaking world. The project is available online; an equivalent print encyclopedia would require roughly 20 volumes.
• Printed books – There are efforts to put a select subset of Wikipedia's articles into printed book form. Since 2009, tens of thousands of print-on-demand books that reproduced English, German, Russian and French Wikipedia articles have been produced by the American company Books LLC and by three Mauritian subsidiaries of the German publisher VDM.
• Semantic Web – The website DBpedia, begun in 2007, extracts data from the infoboxes and category declarations of the English-language Wikipedia. Wikimedia has created the Wikidata project with a similar objective of storing the basic facts from each page of Wikipedia and the other WMF wikis and make it available in a queriable semantic format, RDF. This is still under development. As of February 2014 it has 15,000,000 items and 1,000 properties for describing them.

Obtaining the full contents of Wikipedia for reuse presents challenges, since direct cloning via a web crawler is discouraged. Wikipedia publishes "dumps" of its contents, but these are text-only; as of 2007 there was no dump available of Wikipedia's images.

Several languages of Wikipedia also maintain a reference desk, where volunteers answer questions from the general public. According to a study by Pnina Shachaf in the Journal of Documentation, the quality of the Wikipedia reference desk is comparable to a standard library reference desk, with an accuracy of 55%.

Mobile access

The mobile version of the English Wikipedia's main page

Wikipedia's original medium was for users to read and edit content using any standard web browser through a fixed Internet connection. Although Wikipedia content has been accessible through the mobile web since July 2013, The New York Times on February 9, 2014, quoted Erik Möller, deputy director of the Wikimedia Foundation, stating that the transition of internet traffic from desktops to mobile devices was significant and a cause for concern and worry. The article in The New York Times reported the comparison statistics for mobile edits stating that, "Only 20 percent of the readership of the English-language Wikipedia comes via mobile devices, a figure substantially lower than the percentage of mobile traffic for other media sites, many of which approach 50 percent. And the shift to mobile editing has lagged even more." The New York Times reports that Möller has assigned "a team of 10 software developers focused on mobile", out of a total of approximately 200 employees working at the Wikimedia Foundation. One principal concern cited by The New York Times for the "worry" is for Wikipedia to effectively address attrition issues with the number of editors which the online encyclopedia attracts to edit and maintain its content in a mobile access environment.

Bloomberg Businessweek reported in July 2014 that Google's Android mobile apps have dominated the largest share of global smartphone shipments for 2013 with 78.6% of market share over their next closest competitor in iOS with 15.2% of the market. At the time of the Tretikov appointment and her posted web interview with Sue Gardner in May 2014, Wikimedia representatives made a technical announcement concerning the number of mobile access systems in the market seeking access to Wikipedia. Directly after the posted web interview, the representatives stated that Wikimedia would be applying an all-inclusive approach to accommodate as many mobile access systems as possible in its efforts for expanding general mobile access, including BlackBerry and the Windows Phone system, making market share a secondary issue. The latest version of the Android app for Wikipedia was released on July 23, 2014, to generally positive reviews, scoring over four of a possible five in a poll of approximately 200,000 users downloading from Google. The latest version for iOS was released on April 3, 2013, to similar reviews.

Access to Wikipedia from mobile phones was possible as early as 2004, through the Wireless Application Protocol (WAP), via the Wapedia service. In June 2007 Wikipedia launched en.mobile.wikipedia.org, an official website for wireless devices. In 2009 a newer mobile service was officially released, located at en.m.wikipedia.org, which caters to more advanced mobile devices such as the iPhone, Android-based devices or WebOS-based devices. Several other methods of mobile access to Wikipedia have emerged. Many devices and applications optimize or enhance the display of Wikipedia content for mobile devices, while some also incorporate additional features such as use of Wikipedia metadata, such as geoinformation.

Wikipedia Zero is an initiative of the Wikimedia Foundation to expand the reach of the encyclopedia to the developing countries.

Andrew Lih and Andrew Brown both maintain editing Wikipedia with smart phones is difficult and this discourages new potential contributors. Several years running the number of Wikipedia editors has been falling and Tom Simonite of MIT Technology Review claims the bureaucratic structure and rules are a factor in this. Simonite alleges some Wikipedians use the labyrinthine rules and guidelines to dominate others and those editors have a vested interest in keeping the status quo. Lih alleges there is serious disagreement among existing contributors how to resolve this. Lih fears for Wikipedia's long term future while Brown fears problems with Wikipedia will remain and rival encyclopedias will not replace it.

Cultural impact

Trusted source to combat fake news

In 2017–18, after a barrage of false news reports, both Facebook and YouTube announced they would rely on Wikipedia to help their users evaluate reports and reject false news. Noam Cohen, writing in The Washington Post states, "YouTube’s reliance on Wikipedia to set the record straight builds on the thinking of another fact-challenged platform, the Facebook social network, which announced last year that Wikipedia would help its users root out 'fake news'." In answer to the question of 'how engaged are visitors to the site?' Alexa records the daily page views per visitor as 3.10 and the daily time on site as 4.11 minutes.

Readership

Wikipedia is extremely popular. In February 2014, The New York Times reported that Wikipedia is ranked fifth globally among all websites, stating "With 18 billion page views and nearly 500 million unique visitors a month [...] Wikipedia trails just Yahoo, Facebook, Microsoft and Google, the largest with 1.2 billion unique visitors."

In addition to logistic growth in the number of its articles, Wikipedia has steadily gained status as a general reference website since its inception in 2001. About 50% of search engine traffic to Wikipedia comes from Google, a good portion of which is related to academic research. The number of readers of Wikipedia worldwide reached 365 million at the end of 2009. The Pew Internet and American Life project found that one third of US Internet users consulted Wikipedia. In 2011 Business Insider gave Wikipedia a valuation of $4 billion if it ran advertisements.

According to "Wikipedia Readership Survey 2011", the average age of Wikipedia readers is 36, with a rough parity between genders. Almost half of Wikipedia readers visit the site more than five times a month, and a similar number of readers specifically look for Wikipedia in search engine results. About 47% of Wikipedia readers do not realize that Wikipedia is a non-profit organization.

Cultural significance

Wikipedia Monument in Słubice, Poland

Wikipedia's content has also been used in academic studies, books, conferences, and court cases. The Parliament of Canada's website refers to Wikipedia's article on same-sex marriage in the "related links" section of its "further reading" list for the Civil Marriage Act. The encyclopedia's assertions are increasingly used as a source by organizations such as the US federal courts and the World Intellectual Property Organization – though mainly for supporting information rather than information decisive to a case. Content appearing on Wikipedia has also been cited as a source and referenced in some US intelligence agency reports. In December 2008, the scientific journal RNA Biology launched a new section for descriptions of families of RNA molecules and requires authors who contribute to the section to also submit a draft article on the RNA family for publication in Wikipedia.

Wikipedia has also been used as a source in journalism, often without attribution, and several reporters have been dismissed for plagiarizing from Wikipedia.

In 2006, Time magazine recognized Wikipedia's participation (along with YouTube, Reddit, MySpace, and Facebook) in the rapid growth of online collaboration and interaction by millions of people worldwide.

In July 2007 Wikipedia was the focus of a 30-minute documentary on BBC Radio 4 which argued that, with increased usage and awareness, the number of references to Wikipedia in popular culture is such that the word is one of a select band of 21st-century nouns that are so familiar (Google, Facebook, YouTube) that they no longer need explanation.

On September 28, 2007, Italian politician Franco Grillini raised a parliamentary question with the minister of cultural resources and activities about the necessity of freedom of panorama. He said that the lack of such freedom forced Wikipedia, "the seventh most consulted website", to forbid all images of modern Italian buildings and art, and claimed this was hugely damaging to tourist revenues.

Jimmy Wales receiving the Quadriga A Mission of Enlightenment award

On September 16, 2007, The Washington Post reported that Wikipedia had become a focal point in the 2008 US election campaign, saying: "Type a candidate's name into Google, and among the first results is a Wikipedia page, making those entries arguably as important as any ad in defining a candidate. Already, the presidential entries are being edited, dissected and debated countless times each day." An October 2007 Reuters article, titled "Wikipedia page the latest status symbol", reported the recent phenomenon of how having a Wikipedia article vindicates one's notability.

Active participation also has an impact. Law students have been assigned to write Wikipedia articles as an exercise in clear and succinct writing for an uninitiated audience.

A working group led by Peter Stone (formed as a part of the Stanford-based project One Hundred Year Study on Artificial Intelligence) in its report called Wikipedia "the best-known example of crowdsourcing... that far exceeds traditionally-compiled information sources, such as encyclopedias and dictionaries, in scale and depth."

In a 2017 opinion piece for Wired, Hossein Derakhshan describes Wikipedia as "one of the last remaining pillars of the open and decentralized web" and contrasted its existence as a text-based source of knowledge with social media and social networking services, the latter having "since colonized the web for television's values." For Derakhshan, Wikipedia's goal as an encyclopedia represents the Age of Enlightenment tradition of rationality triumphing over emotions, a trend which he considers "endangered" due to the "gradual shift from a typographic culture to a photographic one, which in turn mean[s] a shift from rationality to emotions, exposition to entertainment." Rather than "sapere aude" (lit. 'dare to know'), social networks have led to a culture of "[d]are not to care to know." This is while Wikipedia faces "a more concerning problem" than funding, namely "a flattening growth rate in the number of contributors to the website." Consequently, the challenge for Wikipedia and those who use it is to "save Wikipedia and its promise of a free and open collection of all human knowledge amid the conquest of new and old television—how to collect and preserve knowledge when nobody cares to know."

Awards

Wikipedia team visiting to Parliament of Asturias

Wikipedians meeting after the Asturias awards ceremony

Wikipedia won two major awards in May 2004. The first was a Golden Nica for Digital Communities of the annual Prix Ars Electronica contest; this came with a €10,000 (£6,588; $12,700) grant and an invitation to present at the PAE Cyberarts Festival in Austria later that year. The second was a Judges' Webby Award for the "community" category. Wikipedia was also nominated for a "Best Practices" Webby award. 

In 2007, readers of brandchannel.com voted Wikipedia as the fourth-highest brand ranking, receiving 15% of the votes in answer to the question "Which brand had the most impact on our lives in 2006?"

In September 2008, Wikipedia received Quadriga A Mission of Enlightenment award of Werkstatt Deutschland along with Boris Tadić, Eckart Höfling, and Peter Gabriel. The award was presented to Wales by David Weinberger.

In 2015, Wikipedia was awarded both the annual Erasmus Prize, which recognizes exceptional contributions to culture, society or social sciences, and the Spanish Princess of Asturias Award on International Cooperation. Speaking at the Asturian Parliament in Oviedo, the city that hosts the awards ceremony, Jimmy Wales praised the work of the Asturian language Wikipedia users. The night of the ceremony, members of the Wikimedia Foundation held a meeting with Wikipedians from all parts of Spain, including the local Asturian community.

Satire

Many parodies target Wikipedia's openness and susceptibility to inserted inaccuracies, with characters vandalizing or modifying the online encyclopedia project's articles.

Comedian Stephen Colbert has parodied or referenced Wikipedia on numerous episodes of his show The Colbert Report and coined the related term wikiality, meaning "together we can create a reality that we all agree on—the reality we just agreed on". Another example can be found in "Wikipedia Celebrates 750 Years of American Independence", a July 2006 front-page article in The Onion, as well as the 2010 The Onion article "'L.A. Law' Wikipedia Page Viewed 874 Times Today".

In an episode of the television comedy The Office U.S., which aired in April 2007, an incompetent office manager (Michael Scott) is shown relying on a hypothetical Wikipedia article for information on negotiation tactics in order to assist him in negotiating lesser pay for an employee. Viewers of the show tried to add the episode's mention of the page as a section of the actual Wikipedia article on negotiation, but this effort was prevented by other users on the article's talk page.

"My Number One Doctor", a 2007 episode of the television show Scrubs, played on the perception that Wikipedia is an unreliable reference tool with a scene in which Dr. Perry Cox reacts to a patient who says that a Wikipedia article indicates that the raw food diet reverses the effects of bone cancer by retorting that the same editor who wrote that article also wrote the Battlestar Galactica episode guide.

In 2008, the comedic website CollegeHumor produced a video sketch named "Professor Wikipedia", in which the fictitious Professor Wikipedia instructs a class with a medley of unverifiable and occasionally absurd statements.

The Dilbert comic strip from May 8, 2009, features a character supporting an improbable claim by saying "Give me ten minutes and then check Wikipedia."

In July 2009, BBC Radio 4 broadcast a comedy series called Bigipedia, which was set on a website which was a parody of Wikipedia. Some of the sketches were directly inspired by Wikipedia and its articles.

In 2010, comedian Daniel Tosh encouraged viewers of his show, Tosh.0, to visit the show's Wikipedia article and edit it at will. On a later episode, he commented on the edits to the article, most of them offensive, which had been made by the audience and had prompted the article to be locked from editing.

On August 23, 2013, the New Yorker website published a cartoon with this caption: "Dammit, Manning, have you considered the pronoun war that this is going to start on your Wikipedia page?" The cartoon referred to Chelsea Elizabeth Manning (born Bradley Edward Manning), an American activist, politician, and former United States Army soldier and a trans woman.

In December 2015, John Julius Norwich stated, in a letter published in The Times newspaper, that as an historian he resorted to Wikipedia "at least a dozen times a day", and had never yet caught it out. He described it as "a work of reference as useful as any in existence", with so wide a range that it is almost impossible to find a person, place or thing that it has left uncovered, and that he could never have written his last two books without it.

Sister projects – Wikimedia

Wikipedia has also spawned several sister projects, which are also wikis run by the Wikimedia Foundation. These other Wikimedia projects include Wiktionary, a dictionary project launched in December 2002, Wikiquote, a collection of quotations created a week after Wikimedia launched, Wikibooks, a collection of collaboratively written free textbooks and annotated texts, Wikimedia Commons, a site devoted to free-knowledge multimedia, Wikinews, for citizen journalism, and Wikiversity, a project for the creation of free learning materials and the provision of online learning activities. Another sister project of Wikipedia, Wikispecies, is a catalogue of species. In 2012 Wikivoyage, an editable travel guide, and Wikidata, an editable knowledge base, launched.

Publishing

A group of Wikimedians of the Wikimedia DC chapter at the 2013 DC Wikimedia annual meeting standing in front of the Encyclopædia Britannica (back left) at the US National Archives

The most obvious economic effect of Wikipedia has been the death of commercial encyclopedias, especially the printed versions, e.g. Encyclopædia Britannica, which were unable to compete with a product that is essentially free. Nicholas Carr wrote a 2005 essay, "The amorality of Web 2.0", that criticized websites with user-generated content, like Wikipedia, for possibly leading to professional (and, in his view, superior) content producers' going out of business, because "free trumps quality all the time". Carr wrote: "Implicit in the ecstatic visions of Web 2.0 is the hegemony of the amateur. I for one can't imagine anything more frightening." Others dispute the notion that Wikipedia, or similar efforts, will entirely displace traditional publications. For instance, Chris Anderson, the editor-in-chief of Wired Magazine, wrote in Nature that the "wisdom of crowds" approach of Wikipedia will not displace top scientific journals, with their rigorous peer review process.

There is also an ongoing debate about the influence of Wikipedia on the biography publishing business. "The worry is that, if you can get all that information from Wikipedia, what's left for biography?" said Kathryn Hughes, professor of life writing at UEA and author of The Short Life and Long Times of Mrs Beeton and George Eliot: the Last Victorian.

Research use

Wikipedia has been widely used as a corpus for linguistic research in computational linguistics, information retrieval and natural language processing. In particular, it commonly serves as a target knowledge base for the entity linking problem, which is then called "wikification", and to the related problem of word sense disambiguation. Methods similar to wikification can in turn be used to find "missing" links in Wikipedia.

In 2015, French researchers Dr José Lages of the University of Franche-Comté in Besançon and Dima Shepelyansky of Paul Sabatier University in Toulouse published a global university ranking based on Wikipedia scholarly citations. They used PageRank "followed by the number of appearances in the 24 different language editions of Wikipedia (descending order) and the century in which they were founded (ascending order)."

A 2017 MIT study suggests that words used on Wikipedia articles end up in scientific publications.

Studies related to Wikipedia has been using machine learning and artificial intelligence to support various operations. One of the most important areas - automatic detection of vandalism and data quality assessment in Wikipedia, which may include different measures for articles and infoboxes.

Related projects

A number of interactive multimedia encyclopedias incorporating entries written by the public existed long before Wikipedia was founded. The first of these was the 1986 BBC Domesday Project, which included text (entered on BBC Micro computers) and photographs from over 1 million contributors in the UK, and covered the geography, art, and culture of the UK. This was the first interactive multimedia encyclopedia (and was also the first major multimedia document connected through internal links), with the majority of articles being accessible through an interactive map of the UK. The user interface and part of the content of the Domesday Project were emulated on a website until 2008.

Several free-content, collaborative encyclopedias were created around the same period as Wikipedia (e.g. Everything2), with many later being merged into the project (e.g. GNE). One of the most successful early online encyclopedias incorporating entries by the public was h2g2, which was created by Douglas Adams in 1999. The h2g2 encyclopedia is relatively light-hearted, focusing on articles which are both witty and informative. 

Subsequent collaborative knowledge websites have drawn inspiration from Wikipedia. Some, such as Susning.nu, Enciclopedia Libre, Hudong, and Baidu Baike likewise employ no formal review process, although some like Conservapedia are not as open. Others use more traditional peer review, such as Encyclopedia of Life and the online wiki encyclopedias Scholarpedia and Citizendium. The latter was started by Sanger in an attempt to create a reliable alternative to Wikipedia.

Knowledge representation and reasoning

From Wikipedia, the free encyclopedia

Knowledge representation and reasoning (KR, KR², KR&R) is the field of artificial intelligence (AI) dedicated to representing information about the world in a form that a computer system can utilize to solve complex tasks such as diagnosing a medical condition or having a dialog in a natural language. Knowledge representation incorporates findings from psychology about how humans solve problems and represent knowledge in order to design formalisms that will make complex systems easier to design and build. Knowledge representation and reasoning also incorporates findings from logic to automate various kinds of reasoning, such as the application of rules or the relations of sets and subsets.

Examples of knowledge representation formalisms include semantic nets, systems architecture, frames, rules, and ontologies. Examples of automated reasoning engines include inference engines, theorem provers, and classifiers.

The KR conference series was established to share ideas and progress on this challenging field.

History

The earliest work in computerized knowledge representation was focused on general problem solvers such as the General Problem Solver (GPS) system developed by Allen Newell and Herbert A. Simon in 1959. These systems featured data structures for planning and decomposition. The system would begin with a goal. It would then decompose that goal into sub-goals and then set out to construct strategies that could accomplish each subgoal. 

In these early days of AI, general search algorithms such as A* were also developed. However, the amorphous problem definitions for systems such as GPS meant that they worked only for very constrained toy domains (e.g. the "blocks world"). In order to tackle non-toy problems, AI researchers such as Ed Feigenbaum and Frederick Hayes-Roth realized that it was necessary to focus systems on more constrained problems. 

It was the failure of these efforts that led to the cognitive revolution in psychology and to the phase of AI focused on knowledge representation that resulted in expert systems in the 1970s and 80s, production systems, frame languages, etc. Rather than general problem solvers, AI changed its focus to expert systems that could match human competence on a specific task, such as medical diagnosis.

Expert systems gave us the terminology still in use today where AI systems are divided into a Knowledge Base with facts about the world and rules and an inference engine that applies the rules to the knowledge base in order to answer questions and solve problems. In these early systems the knowledge base tended to be a fairly flat structure, essentially assertions about the values of variables used by the rules.

In addition to expert systems, other researchers developed the concept of frame based languages in the mid 1980s. A frame is similar to an object class: It is an abstract description of a category describing things in the world, problems, and potential solutions. Frames were originally used on systems geared toward human interaction, e.g. understanding natural language and the social settings in which various default expectations such as ordering food in a restaurant narrow the search space and allow the system to choose appropriate responses to dynamic situations.

It wasn't long before the frame communities and the rule-based researchers realized that there was synergy between their approaches. Frames were good for representing the real world, described as classes, subclasses, slots (data values) with various constraints on possible values. Rules were good for representing and utilizing complex logic such as the process to make a medical diagnosis. Integrated systems were developed that combined Frames and Rules. One of the most powerful and well known was the 1983 Knowledge Engineering Environment (KEE) from Intellicorp. KEE had a complete rule engine with forward and backward chaining. It also had a complete frame based knowledge base with triggers, slots (data values), inheritance, and message passing. Although message passing originated in the object-oriented community rather than AI it was quickly embraced by AI researchers as well in environments such as KEE and in the operating systems for Lisp machines from Symbolics, Xerox, and Texas Instruments.

The integration of Frames, rules, and object-oriented programming was significantly driven by commercial ventures such as KEE and Symbolics spun off from various research projects. At the same time as this was occurring, there was another strain of research which was less commercially focused and was driven by mathematical logic and automated theorem proving. One of the most influential languages in this research was the KL-ONE language of the mid 80's. KL-ONE was a frame language that had a rigorous semantics, formal definitions for concepts such as an Is-A relation. KL-ONE and languages that were influenced by it such as Loom had an automated reasoning engine that was based on formal logic rather than on IF-THEN rules. This reasoner is called the classifier. A classifier can analyze a set of declarations and infer new assertions, for example, redefine a class to be a subclass or superclass of some other class that wasn't formally specified. In this way the classifier can function as an inference engine, deducing new facts from an existing knowledge base. The classifier can also provide consistency checking on a knowledge base (which in the case of KL-ONE languages is also referred to as an Ontology).

Another area of knowledge representation research was the problem of common sense reasoning. One of the first realizations learned from trying to make software that can function with human natural language was that humans regularly draw on an extensive foundation of knowledge about the real world that we simply take for granted but that is not at all obvious to an artificial agent. Basic principles of common sense physics, causality, intentions, etc. An example is the frame problem, that in an event driven logic there need to be axioms that state things maintain position from one moment to the next unless they are moved by some external force. In order to make a true artificial intelligence agent that can converse with humans using natural language and can process basic statements and questions about the world, it is essential to represent this kind of knowledge. One of the most ambitious programs to tackle this problem was Doug Lenat's Cyc project. Cyc established its own Frame language and had large numbers of analysts document various areas of common sense reasoning in that language. The knowledge recorded in Cyc included common sense models of time, causality, physics, intentions, and many others.

The starting point for knowledge representation is the knowledge representation hypothesis first formalized by Brian C. Smith in 1985:

Any mechanically embodied intelligent process will be comprised of structural ingredients that a) we as external observers naturally take to represent a propositional account of the knowledge that the overall process exhibits, and b) independent of such external semantic attribution, play a formal but causal and essential role in engendering the behavior that manifests that knowledge.

Currently one of the most active areas of knowledge representation research are projects associated with the semantic web. The semantic web seeks to add a layer of semantics (meaning) on top of the current Internet. Rather than indexing web sites and pages via keywords, the semantic web creates large ontologies of concepts. Searching for a concept will be more effective than traditional text only searches. Frame languages and automatic classification play a big part in the vision for the future semantic web. The automatic classification gives developers technology to provide order on a constantly evolving network of knowledge. Defining ontologies that are static and incapable of evolving on the fly would be very limiting for Internet-based systems. The classifier technology provides the ability to deal with the dynamic environment of the Internet.

Recent projects funded primarily by the Defense Advanced Research Projects Agency (DARPA) have integrated frame languages and classifiers with markup languages based on XML. The Resource Description Framework (RDF) provides the basic capability to define classes, subclasses, and properties of objects. The Web Ontology Language (OWL) provides additional levels of semantics and enables integration with classification engines.

Overview

Knowledge-representation is the field of artificial intelligence that focuses on designing computer representations that capture information about the world that can be used to solve complex problems.

The justification for knowledge representation is that conventional procedural code is not the best formalism to use to solve complex problems. Knowledge representation makes complex software easier to define and maintain than procedural code and can be used in expert systems.

For example, talking to experts in terms of business rules rather than code lessens the semantic gap between users and developers and makes development of complex systems more practical. 

Knowledge representation goes hand in hand with automated reasoning because one of the main purposes of explicitly representing knowledge is to be able to reason about that knowledge, to make inferences, assert new knowledge, etc. Virtually all knowledge representation languages have a reasoning or inference engine as part of the system.

A key trade-off in the design of a knowledge representation formalism is that between expressivity and practicality. The ultimate knowledge representation formalism in terms of expressive power and compactness is First Order Logic (FOL). There is no more powerful formalism than that used by mathematicians to define general propositions about the world. However, FOL has two drawbacks as a knowledge representation formalism: ease of use and practicality of implementation. First order logic can be intimidating even for many software developers. Languages which do not have the complete formal power of FOL can still provide close to the same expressive power with a user interface that is more practical for the average developer to understand. The issue of practicality of implementation is that FOL in some ways is too expressive. With FOL it is possible to create statements (e.g. quantification over infinite sets) that would cause a system to never terminate if it attempted to verify them. 

Thus, a subset of FOL can be both easier to use and more practical to implement. This was a driving motivation behind rule-based expert systems. IF-THEN rules provide a subset of FOL but a very useful one that is also very intuitive. The history of most of the early AI knowledge representation formalisms; from databases to semantic nets to theorem provers and production systems can be viewed as various design decisions on whether to emphasize expressive power or computability and efficiency.

In a key 1993 paper on the topic, Randall Davis of MIT outlined five distinct roles to analyze a knowledge representation framework:

• A knowledge representation (KR) is most fundamentally a surrogate, a substitute for the thing itself, used to enable an entity to determine consequences by thinking rather than acting, i.e., by reasoning about the world rather than taking action in it.
• It is a set of ontological commitments, i.e., an answer to the question: In what terms should I think about the world?
• It is a fragmentary theory of intelligent reasoning, expressed in terms of three components: (i) the representation's fundamental conception of intelligent reasoning; (ii) the set of inferences the representation sanctions; and (iii) the set of inferences it recommends.
• It is a medium for pragmatically efficient computation, i.e., the computational environment in which thinking is accomplished. One contribution to this pragmatic efficiency is supplied by the guidance a representation provides for organizing information so as to facilitate making the recommended inferences.
• It is a medium of human expression, i.e., a language in which we say things about the world.

Knowledge representation and reasoning are a key enabling technology for the Semantic web. Languages based on the Frame model with automatic classification provide a layer of semantics on top of the existing Internet. Rather than searching via text strings as is typical today, it will be possible to define logical queries and find pages that map to those queries. The automated reasoning component in these systems is an engine known as the classifier. Classifiers focus on the subsumption relations in a knowledge base rather than rules. A classifier can infer new classes and dynamically change the ontology as new information becomes available. This capability is ideal for the ever-changing and evolving information space of the Internet.

The Semantic web integrates concepts from knowledge representation and reasoning with markup languages based on XML. The Resource Description Framework (RDF) provides the basic capabilities to define knowledge-based objects on the Internet with basic features such as Is-A relations and object properties. The Web Ontology Language (OWL) adds additional semantics and integrates with automatic classification reasoners.

Characteristics

In 1985, Ron Brachman categorized the core issues for knowledge representation as follows:

• Primitives. What is the underlying framework used to represent knowledge? Semantic networks were one of the first knowledge representation primitives. Also, data structures and algorithms for general fast search. In this area, there is a strong overlap with research in data structures and algorithms in computer science. In early systems, the Lisp programming language, which was modeled after the lambda calculus, was often used as a form of functional knowledge representation. Frames and Rules were the next kind of primitive. Frame languages had various mechanisms for expressing and enforcing constraints on frame data. All data in frames are stored in slots. Slots are analogous to relations in entity-relation modeling and to object properties in object-oriented modeling. Another technique for primitives is to define languages that are modeled after First Order Logic (FOL). The most well known example is Prolog, but there are also many special purpose theorem proving environments. These environments can validate logical models and can deduce new theories from existing models. Essentially they automate the process a logician would go through in analyzing a model. Theorem proving technology had some specific practical applications in the areas of software engineering. For example, it is possible to prove that a software program rigidly adheres to a formal logical specification.
• Meta-representation. This is also known as the issue of reflection in computer science. It refers to the capability of a formalism to have access to information about its own state. An example would be the meta-object protocol in Smalltalk and CLOS that gives developers run time access to the class objects and enables them to dynamically redefine the structure of the knowledge base even at run time. Meta-representation means the knowledge representation language is itself expressed in that language. For example, in most Frame based environments all frames would be instances of a frame class. That class object can be inspected at run time, so that the object can understand and even change its internal structure or the structure of other parts of the model. In rule-based environments, the rules were also usually instances of rule classes. Part of the meta protocol for rules were the meta rules that prioritized rule firing.
• Incompleteness. Traditional logic requires additional axioms and constraints to deal with the real world as opposed to the world of mathematics. Also, it is often useful to associate degrees of confidence with a statement. I.e., not simply say "Socrates is Human" but rather "Socrates is Human with confidence 50%". This was one of the early innovations from expert systems research which migrated to some commercial tools, the ability to associate certainty factors with rules and conclusions. Later research in this area is known as fuzzy logic.
• Definitions and universals vs. facts and defaults. Universals are general statements about the world such as "All humans are mortal". Facts are specific examples of universals such as "Socrates is a human and therefore mortal". In logical terms definitions and universals are about universal quantification while facts and defaults are about existential quantifications. All forms of knowledge representation must deal with this aspect and most do so with some variant of set theory, modeling universals as sets and subsets and definitions as elements in those sets.
• Non-monotonic reasoning. Non-monotonic reasoning allows various kinds of hypothetical reasoning. The system associates facts asserted with the rules and facts used to justify them and as those facts change updates the dependent knowledge as well. In rule based systems this capability is known as a truth maintenance system.
• Expressive adequacy. The standard that Brachman and most AI researchers use to measure expressive adequacy is usually First Order Logic (FOL). Theoretical limitations mean that a full implementation of FOL is not practical. Researchers should be clear about how expressive (how much of full FOL expressive power) they intend their representation to be.
• Reasoning efficiency. This refers to the run time efficiency of the system. The ability of the knowledge base to be updated and the reasoner to develop new inferences in a reasonable period of time. In some ways, this is the flip side of expressive adequacy. In general, the more powerful a representation, the more it has expressive adequacy, the less efficient its automated reasoning engine will be. Efficiency was often an issue, especially for early applications of knowledge representation technology. They were usually implemented in interpreted environments such as Lisp, which were slow compared to more traditional platforms of the time.

Ontology engineering

In the early years of knowledge-based systems the knowledge-bases were fairly small. The knowledge-bases that were meant to actually solve real problems rather than do proof of concept demonstrations needed to focus on well defined problems. So for example, not just medical diagnosis as a whole topic, but medical diagnosis of certain kinds of diseases. 

As knowledge-based technology scaled up, the need for larger knowledge bases and for modular knowledge bases that could communicate and integrate with each other became apparent. This gave rise to the discipline of ontology engineering, designing and building large knowledge bases that could be used by multiple projects. One of the leading research projects in this area was the Cyc project. Cyc was an attempt to build a huge encyclopedic knowledge base that would contain not just expert knowledge but common sense knowledge. In designing an artificial intelligence agent, it was soon realized that representing common sense knowledge, knowledge that humans simply take for granted, was essential to make an AI that could interact with humans using natural language. Cyc was meant to address this problem. The language they defined was known as CycL. 

After CycL, a number of ontology languages have been developed. Most are declarative languages, and are either frame languages, or are based on first-order logic. Modularity—the ability to define boundaries around specific domains and problem spaces—is essential for these languages because as stated by Tom Gruber, "Every ontology is a treaty- a social agreement among people with common motive in sharing." There are always many competing and differing views that make any general purpose ontology impossible. A general purpose ontology would have to be applicable in any domain and different areas of knowledge need to be unified.

There is a long history of work attempting to build ontologies for a variety of task domains, e.g., an ontology for liquids, the lumped element model widely used in representing electronic circuits, e.g., as well as ontologies for time, belief, and even programming itself. Each of these offers a way to see some part of the world. 

The lumped element model, for instance, suggests that we think of circuits in terms of components with connections between them, with signals flowing instantaneously along the connections. This is a useful view, but not the only possible one. A different ontology arises if we need to attend to the electrodynamics in the device: Here signals propagate at finite speed and an object (like a resistor) that was previously viewed as a single component with an I/O behavior may now have to be thought of as an extended medium through which an electromagnetic wave flows. 

Ontologies can of course be written down in a wide variety of languages and notations (e.g., logic, LISP, etc.); the essential information is not the form of that language but the content, i.e., the set of concepts offered as a way of thinking about the world. Simply put, the important part is notions like connections and components, not the choice between writing them as predicates or LISP constructs. 

The commitment made selecting one or another ontology can produce a sharply different view of the task at hand. Consider the difference that arises in selecting the lumped element view of a circuit rather than the electrodynamic view of the same device. As a second example, medical diagnosis viewed in terms of rules (e.g., MYCIN) looks substantially different from the same task viewed in terms of frames (e.g., INTERNIST). Where MYCIN sees the medical world as made up of empirical associations connecting symptom to disease, INTERNIST sees a set of prototypes, in particular prototypical diseases, to be matched against the case at hand.