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Tuesday, February 18, 2025

Nomad

From Wikipedia, the free encyclopedia
A painting by Vincent van Gogh depicting a caravan of nomadic Roma

Nomadic hunting and gathering—following seasonally available wild plants and game—is by far the oldest human subsistence method. Pastoralists raise herds of domesticated livestock, driving or accompanying them in patterns that normally avoid depleting pastures beyond their ability to recover. Nomadism is also a lifestyle adapted to infertile regions such as steppe, tundra, or ice and sand, where mobility is the most efficient strategy for exploiting scarce resources. For example, many groups living in the tundra are reindeer herders and are semi-nomadic, following forage for their animals.

Sometimes also described as "nomadic" are various itinerant populations who move among densely populated areas to offer specialized services (crafts or trades) to their residents—external consultants, for example. These groups are known as "peripatetic nomads".

Etymology

The English word nomad comes from the Middle French nomade, from Latin nomas ("wandering shepherd"), from Ancient Greek νομᾰ́ς (nomás, “roaming, wandering, esp. to find pasture”), which is derived from the Ancient Greek νομός (nomós, “pasture”).

Common characteristics

Roma mother and child
Nomads on the Changtang, Ladakh
Rider in Mongolia, 2012. While nomadic life is less common in modern times, the horse remains a national symbol in Mongolia.
Beja nomads from Northeast Africa

Nomads are communities who move from place to place as a way of obtaining food, finding pasture for livestock, or otherwise making a living. Most nomadic groups follow a fixed annual or seasonal pattern of movements and settlements. Nomadic people traditionally travel by animal, canoe or on foot. Animals include camels, horses and alpaca. Today, some nomads travel by motor vehicle. Some nomads may live in homes or homeless shelters, though this would necessarily be on a temporary or itinerant basis.

Nomads keep moving for different reasons. Nomadic foragers move in search of game, edible plants, and water. Aboriginal Australians, Negritos of Southeast Asia, and San of Africa, for example, traditionally move from camp to camp to hunt and gather wild plants. Some tribes of the Americas followed this way of life. Pastoral nomads, on the other hand, make their living raising livestock such as camels, cattle, goats, horses, sheep, or yaks; these nomads usually travel in search of pastures for their flocks. The Fulani and their cattle travel through the grasslands of Niger in western Africa. Some nomadic peoples, especially herders, may also move to raid settled communities or to avoid enemies. Nomadic craftworkers and merchants travel to find and serve customers. They include the Gadia Lohar blacksmiths of India, the Roma traders, Scottish travellers and Irish travellers.

Many nomadic and pastorally nomadic peoples are associated with semi-arid and desert climates; examples include the Mongolic and Turkic peoples of Central Asia, the Plains Indians of the Great Plains, and the Amazigh and other peoples of the Sahara Desert. Pastoral nomads who are residents of arid climates include the Fulani of the Sahel, the Khoikhoi of South Africa and Namibia, groups of Northeast Africa such as Somalis and Oromo, and the Bedouin of the Middle East.

Most nomads travel in groups of families, bands, or tribes. These groups are based on kinship and marriage ties or on formal agreements of cooperation. A council of adult males makes most of the decisions, though some tribes have chiefs.

In the case of Mongolian nomads, a family moves twice a year. These two movements generally occur during the summer and winter. The winter destination is usually located near the mountains in a valley and most families already have fixed winter locations. Their winter locations have shelter for animals and are not used by other families while they are out. In the summer they move to a more open area in which the animals can graze. Most nomads usually move within the same region and do not travel very far. Since they usually circle around a large area, communities form and families generally know where the other ones are. Often, families do not have the resources to move from one province to another unless they are moving out of the area permanently. A family can move on its own or with others; if it moves alone, they are usually no more than a couple of kilometres from each other. The geographical closeness of families is usually for mutual support. Pastoral nomad societies usually do not have large populations.

One nomadic society, the Mongols, gave rise to the largest land empire in history. The Mongols originally consisted of loosely organized nomadic tribes in Mongolia, Manchuria, and Siberia. In the late 12th century, Genghis Khan united them and other nomadic tribes to found the Mongol Empire, which eventually stretched the length of Asia.

The nomadic way of life has become increasingly rare. Many countries have converted pastures into cropland and forced nomadic peoples into permanent settlements.

Modern forms of nomadic peoples are variously referred to as "shiftless", "gypsies", "rootless cosmopolitans", hunter-gatherers, refugees and urban homeless or street-people, depending on their individual circumstances. These terms may be used in a derogatory sense.

According to Gérard Chaliand, terrorism originated in nomad-warrior cultures. He points to Machiavelli's classification of war into two types, which Chaliand interprets as describing a difference between warfare in sedentary and nomadic societies:

There are two different kinds of war. The one springs from the ambition of princes or republics that seek to extend their empire; such were the wars of Alexander the Great, and those of the Romans, and those which two hostile powers carry on against each other. These wars are dangerous but never go so far as to drive all its inhabitants out of a province, because the conqueror is satisfied with the submission of the people... The other kind of war is when an entire people, constrained by famine or war, leave their country with their families for the purpose of seeking a new home in a new country, not for the purpose of subjecting it to their dominion as in the first case, but with the intention of taking absolute possession of it themselves and driving out or killing its original inhabitants.

Primary historical sources for nomadic steppe-style warfare are found in many languages: Chinese, Persian, Polish, Russian, Classical Greek, Armenian, Latin and Arabic. These sources concern both the true steppe nomads (Mongols, Huns, Magyars and Scythians) and also the semi-settled people like Turks, Crimean Tatars and Russians, who retained or, in some cases, adopted the nomadic form of warfare.

Hunter-gatherers

Starting fire by hand. San people in Botswana.

Hunter-gatherers (also known as foragers) move from campsite to campsite, following game and wild fruits and vegetables. Hunting and gathering describes early peoples' subsistence living style. Following the development of agriculture, most hunter-gatherers were eventually either displaced or converted to farming or pastoralist groups. Only a few contemporary societies, such as the Pygmies, the Hadza people, and some uncontacted tribes in the Amazon rainforest, are classified as hunter-gatherers; some of these societies supplement, sometimes extensively, their foraging activity with farming or animal husbandry.

Pastoralism

Overview map of the world in 200 BC:
  Sarmatians, Saka, Yuezhi, Xiongnu and other nomadic pastoralists
Cuman nomads, Radziwiłł Chronicle, 13th century.
A yurt in front of the Gurvan Saikhan Mountains. Approximately 30% of Mongolia's 3 million people are nomadic or semi-nomadic.
A Sámi family in Norway around 1900. Reindeer have been herded for centuries by several Arctic and Subarctic people including the Sámi and the Nenets.

Pastoral nomads are nomads moving between pastures. Nomadic pastoralism is thought to have developed in three stages that accompanied population growth and an increase in the complexity of social organization. Karim Sadr has proposed the following stages:

  • Pastoralism: This is a mixed economy with a symbiosis within the family.
  • Agropastoralism: This is when symbiosis is between segments or clans within an ethnic group.
  • True Nomadism: This is when symbiosis is at the regional level, generally between specialised nomadic and agricultural populations.

The pastoralists are sedentary to a certain area, as they move between the permanent spring, summer, autumn and winter (or dry and wet season) pastures for their livestock. The nomads moved depending on the availability of resources.

History

Origins

Nomadic pastoralism seems to have developed first as a part of the secondary-products revolution proposed by Andrew Sherratt, in which early pre-pottery Neolithic cultures that had used animals as live meat ("on the hoof") also began using animals for their secondary products, for example: milk and its associated dairy products, wool and other animal hair, hides (and consequently leather), manure (for fuel and fertilizer), and traction.

The first nomadic pastoral society developed in the period from 8,500 to 6,500 BCE in the area of the southern Levant. There, during a period of increasing aridity, Pre-Pottery Neolithic B (PPNB) cultures in the Sinai were replaced by a nomadic, pastoral pottery-using culture, which seems to have been a cultural fusion between them and a newly-arrived Mesolithic people from Egypt (the Harifian culture), adopting their nomadic hunting lifestyle to the raising of stock.

This lifestyle quickly developed into what Jaris Yurins has called the circum-Arabian nomadic pastoral techno-complex and is possibly associated with the appearance of Semitic languages in the region of the Ancient Near East. The rapid spread of such nomadic pastoralism was typical of such later developments as of the Yamnaya culture of the horse and cattle nomads of the Eurasian steppe (c. 3300–2600 BCE), and of the Mongol spread in the later Middle Ages.

Yamnaya steppe pastoralists from the Pontic–Caspian steppe, who were among the first to master horseback riding, played a key role in Indo-European migrations and in the spread of Indo-European languages across Eurasia.

Trekboers in southern Africa adopted nomadism from the 17th century. Some elements of gaucho culture in colonial South America also re-invented nomadic lifestyles.

Increase in post-Soviet Central Asia

One of the results of the break-up of the Soviet Union and the subsequent political independence and economic collapse of its Central Asian republics has been the resurgence of pastoral nomadism. Taking the Kyrgyz people as a representative example, nomadism was the centre of their economy before Russian colonization at the turn of the 20th century, when they were settled into agricultural villages. The population became increasingly urbanized after World War II, but some people still take their herds of horses and cows to high pastures (jailoo) every summer, continuing a pattern of transhumance.

Since the 1990s, as the cash economy shrank, unemployed relatives were reabsorbed into family farms, and the importance of this form of nomadism has increased.ymbols of nomadism, specifically the crown of the grey felt tent known as the yurt, appears on the national flag, emphasizing the central importance of nomadism in the genesis of the modern nation of Kyrgyzstan.

Sedentarization

From 1920 to 2008, the population of nomadic pastoral tribes slowly decreased from over a quarter of Iran's population. Tribal pastures were nationalized during the 1960s. The National Commission of UNESCO registered the population of Iran at 21 million in 1963, of whom two million (9.5%) were nomads. Although the nomadic population of Iran has dramatically decreased in the 20th century, Iran still has one of the largest nomadic populations in the world, an estimated 1.5 million in a country of about 70 million.

In Kazakhstan where the major agricultural activity was nomadic herding, forced collectivization under Joseph Stalin's rule met with massive resistance and major losses and confiscation of livestock. Livestock in Kazakhstan fell from 7 million cattle to 1.6 million and from 22 million sheep to 1.7 million. The resulting famine of 1931–1934 caused some 1.5 million deaths: this represents more than 40% of the total Kazakh population at that time.

Fulani herdsman in Togo. Spread throughout West Africa, the Fulani are the largest nomadic group in the world.

In the 1950s as well as the 1960s, large numbers of Bedouin throughout the Middle East started to leave the traditional, nomadic life to settle in the cities of the Middle East, especially as home ranges have shrunk and population levels have grown. Government policies in Egypt and Israel, oil production in Libya and the Persian Gulf, as well as a desire for improved standards of living, effectively led most Bedouin to become settled citizens of various nations, rather than stateless nomadic herders. A century ago, nomadic Bedouin still made up some 10% of the total Arab population. Today, they account for some 1% of the total.

At independence in 1960, Mauritania was essentially a nomadic society. The great Sahel droughts of the early 1970s caused massive problems in a country where 85% of its inhabitants were nomadic herders. Today only 15% remain nomads.

As many as 2 million nomadic Kuchis wandered over Afghanistan in the years before the Soviet invasion, and most experts agreed that by 2000 the number had fallen dramatically, perhaps by half. A severe drought had destroyed 80% of the livestock in some areas.

Niger experienced a serious food crisis in 2005 following erratic rainfall and desert locust invasions. Nomads such as the Tuareg and Fulani, who make up about 20% of Niger's 12.9 million population, had been so badly hit by the Niger food crisis that their already fragile way of life is at risk. Nomads in Mali were also affected. The Fulani of West Africa are the world's largest nomadic group.

Lifestyle

Tents of Pashtun nomads in Badghis Province, Afghanistan. They migrate from region to region depending on the season.

Pala nomads living in Western Tibet have a diet that is unusual in that they consume very few vegetables and no fruit. The main staple of their diet is tsampa and they drink Tibetan style butter tea. Pala will eat heartier foods in the winter months to help keep warm. Some of the customary restrictions they explain as cultural saying only that drokha do not eat certain foods, even some that may be naturally abundant. Though they live near sources of fish and fowl these do not play a significant role in their diet, and they do not eat carnivorous animals, rabbits or the wild asses that are abundant in the environs, classifying the latter as horse due to their cloven hooves. Some families do not eat until after the morning milking, while others may have a light meal with butter tea and tsampa. In the afternoon, after the morning milking, the families gather and share a communal meal of tea, tsampa and sometimes yogurt. During winter months the meal is more substantial and includes meat. Herders will eat before leaving the camp and most do not eat again until they return to camp for the evening meal. The typical evening meal may include thin stew with tsampa, animal fat and dried radish. Winter stew would include a lot of meat with either tsampa or boiled flour dumplings.

Nomadic diets in Kazakhstan have not changed much over centuries. The Kazakh nomad cuisine is simple and includes meat, salads, marinated vegetables and fried and baked breads. Tea is served in bowls, possibly with sugar or milk. Milk and other dairy products, like cheese and yogurt, are especially important. Kumys is a drink of fermented milk. Wrestling is a popular sport, but the nomadic people do not have much time for leisure. Horse riding is a valued skill in their culture.

Movement of nomads in Chad

Perception

Ann Marie Kroll Lerner states that the pastoral nomads were viewed as "invading, destructive, and altogether antithetical to civilizing, sedentary societies" during the late 19th and early 20th centuries. According to Lerner, they are rarely accredited as "a civilizing force".

Allan Hill and Sara Randall observe that western authors have looked for "romance and mystery, as well as the repository of laudable characteristics believed lost in the West, such as independence, stoicism in the face of physical adversity, and a strong sense of loyalty to family and to tribe" in nomadic pastoralist societies. Hill and Randall observe that nomadic pastoralists are stereotypically seen by the settled populace in Africa and Middle East as "aimless wanderers, immoral, promiscuous and disease-ridden" peoples. According to Hill and Randall, both of these perceptions "misrepresent the reality".

Contemporary peripatetic minorities in Eurasia

A tent of Romani nomads in Hungary, 19th century.

Peripatetic minorities are mobile populations moving among settled populations offering a craft or trade.

Each existing community is primarily endogamous, and subsists traditionally on a variety of commercial or service activities. Formerly, all or a majority of their members were itinerant, and this largely holds true today. Migration generally takes place within the political boundaries of a single state these days.

Each of the peripatetic communities is multilingual, it speaks one or more of the languages spoken by the local sedentary populations, and, additionally, within each group, a separate dialect or language is spoken. They are speaking languages of Indic origin and many are structured somewhat like an argot or secret language, with vocabularies drawn from various languages. There are indications that in northern Iran at least one community speaks Romani language, and some groups in Turkey also speak Romani.

Asia

Afghanistan

India

Camel grazers in the Thar Desert

Dom people

In Afghanistan, the Nausar worked as tinkers and animal dealers. Ghorbat men mainly made sieves, drums, and bird cages, and the women peddled these as well as other items of household and personal use; they also worked as moneylenders to rural women. Peddling and the sale of various goods was also practiced by men and women of various groups, such as the Jalali, the Pikraj, the Shadibaz, the Noristani, and the Vangawala. The latter and the Pikraj also worked as animal dealers. Some men among the Shadibaz and the Vangawala entertained as monkey or bear handlers and snake charmers; men and women among the Baluch were musicians and dancers. The Baluch men were warriors that were feared by neighboring tribes and often were used as mercenaries. Jogi men and women had diverse subsistence activities, such as dealing in horses, harvesting, fortune-telling, bloodletting, and begging.

In Iran, the Asheq of Azerbaijan, the Challi of Baluchistan, the Luti of Kurdistan, Kermānshāh, Īlām, and Lorestān, the Mehtar in the Mamasani district, the Sazandeh of Band-i Amir and Marv-dasht, and the Toshmal among the Bakhtyari pastoral groups worked as professional musicians. The men among the Kowli worked as tinkers, smiths, musicians, and monkey and bear handlers; they also made baskets, sieves, and brooms and dealt in donkeys. Their women made a living from peddling, begging, and fortune-telling.

The Ghorbat among the Basseri were smiths and tinkers, traded in pack animals, and made sieves, reed mats, and small wooden implements. In the Fārs region, the Qarbalband, the Kuli, and Luli were reported to work as smiths and to make baskets and sieves; they also dealt in pack animals, and their women peddled various goods among pastoral nomads. In the same region, the Changi and Luti were musicians and balladeers, and their children learned these professions from the age of 7 or 8 years.

The nomadic groups in Turkey make and sell cradles, deal in animals, and play music. The men of the sedentary groups work in towns as scavengers and hangmen; elsewhere they are fishermen, smiths, basket makers, and singers; their women dance at feasts and tell fortunes. Abdal men played music and made sieves, brooms, and wooden spoons for a living. The Tahtacı traditionally worked as lumberers; with increased sedentarization, however, they have taken to agriculture and horticulture.

Little is known for certain about the past of these communities; the history of each is almost entirely contained in their oral traditions. Although some groups—such as the Vangawala—are of Indian origin, some—like the Noristani—are most probably of local origin; still others probably migrated from adjoining areas. The Ghorbat and the Shadibaz claim to have originally come from Iran and Multan, respectively, and Tahtacı traditional accounts mention either Baghdad or Khorāsān as their original home. The Baluch say they were attached as a service community to the Jamshedi, after they fled Baluchistan because of feuds.

Kochi people

Yörüks

Still some groups such as Sarıkeçililer continues nomadic lifestyle between coastal towns Mediterranean and Taurus Mountains even though most of them were settled by both late Ottoman and Turkish republic.

Bukat People of Borneo

The Bukat people of Borneo in Malaysia live within the region of the river Mendalam, which the natives call Buköt. Bukat is an ethnonym that encapsulates all the tribes in the region. These natives are historically self-sufficient but were also known to trade various goods. This is especially true for the clans who lived on the periphery of the territory. The products of their trade were varied and fascinating, including: "...resins (damar, Agathis dammara; jelutong bukit, Dyera costulata, gutta-percha, Palaquium spp.); wild honey and beeswax (important in trade but often unreported); aromatic resin from insence wood (gaharu, Aquilaria microcarpa); camphor (found in the fissures of Dryobalanops aromaticus); several types of rotan of cane (Calamus rotan and other species); poison for blowpipe darts (one source is ipoh or ipu: see Nieuwenhuis 1900a:137); the antlers of deer (the sambar, Cervus unicolor); rhinoceros horn (see Tillema 1939:142); pharmacologically valuable bezoar stones (concretions formed in the intestines and gallbladder of the gibbon, Seminopithecus, and in the wounds of porcupines, Hestrix crassispinus); birds' nests, the edible nests of swifts (Collocalia spp.); the heads and feathers of two species of hornbills (Buceros rhinoceros, Rhinoplax vigil); and various hides (clouded leopards, bears, and other animals)." These nomadic tribes also commonly hunted boar with poison blow darts for their own needs.

Encephalitis

From Wikipedia, the free encyclopedia
 
Encephalitis
MRI scan image shows high signal in the temporal lobes and right inferior frontal gyrus in someone with herpes simplex encephalitis.

 
SpecialtyNeurology, infectious disease
SymptomsHeadache, fever, confusion, stiff neck, vomiting
ComplicationsSeizures, trouble speaking, memory problems, problems hearing
DurationWeeks to months for recovery
TypesHerpes simplex, West Nile, rabies, Eastern equine, others
CausesInfection, autoimmune, certain medication, unknown
Diagnostic methodBased on symptoms, supported by blood tests, medical imaging, analysis of cerebrospinal fluid
TreatmentAntiviral medication, anticonvulsants, corticosteroids, artificial respiration
PrognosisVariable
Frequency4.3 million (2015)
Deaths150,000 (2015)

Encephalitis is inflammation of the brain. The severity can be variable with symptoms including reduction or alteration in consciousness, aphasia, headache, fever, confusion, a stiff neck, and vomiting. Complications may include seizures, hallucinations, trouble speaking, memory problems, and problems with hearing.

Causes of encephalitis include viruses such as herpes simplex virus and rabies virus as well as bacteria, fungi, or parasites. Other causes include autoimmune diseases and certain medications. In many cases the cause remains unknown. Risk factors include a weak immune system. Diagnosis is typically based on symptoms and supported by blood tests, medical imaging, and analysis of cerebrospinal fluid.

Certain types are preventable with vaccines. Treatment may include antiviral medications (such as acyclovir), anticonvulsants, and corticosteroids. Treatment generally takes place in hospital. Some people require artificial respiration. Once the immediate problem is under control, rehabilitation may be required. In 2015, encephalitis was estimated to have affected 4.3 million people and resulted in 150,000 deaths worldwide.

Signs and symptoms

Adults with encephalitis present with acute onset of fever, headache, confusion, and sometimes seizures. Younger children or infants may present with irritability, poor appetite and fever. Neurological examinations usually reveal a drowsy or confused person. Stiff neck, due to the irritation of the meninges covering the brain, indicates that the patient has either meningitis or meningoencephalitis.

Limbic encephalitis

Limbic encephalitis refers to inflammatory disease confined to the limbic system of the brain. The clinical presentation often includes disorientation, disinhibition, memory loss, seizures, and behavioral anomalies. MRI imaging reveals T2 hyperintensity in the structures of the medial temporal lobes, and in some cases, other limbic structures. Some cases of limbic encephalitis are of autoimmune origin.

Encephalitis lethargica

Encephalitis lethargica is identified by high fever, headache, delayed physical response, and lethargy. Individuals can exhibit upper body weakness, muscular pains, and tremors, though the cause of encephalitis lethargica is not currently known. From 1917 to 1928, an epidemic of encephalitis lethargica occurred worldwide.

Cause

Rabies virus

In 30%-40% of encephalitis cases, the etiology remains unknown.

Viral

Viral infections are the usual cause of infectious encephalitis. Viral encephalitis can occur either as a direct effect of an acute infection, or as one of the sequelae of a latent infection. The majority of viral cases of encephalitis have an unknown cause; however, the most common identifiable cause of viral encephalitis is from herpes simplex infection. Other causes of acute viral encephalitis are rabies virus, poliovirus, and measles virus.

Additional possible viral causes are arboviral flavivirus (St. Louis encephalitis, West Nile virus), bunyavirus (La Crosse strain), arenavirus (lymphocytic choriomeningitis virus), reovirus (Colorado tick virus), and henipavirus infections. The Powassan virus is a rare cause of encephalitis.

Bacterial

It can be caused by a bacterial infection, such as bacterial meningitis, or may be a complication of a current infectious disease such as syphilis (secondary encephalitis).

Other bacterial pathogens, like Mycoplasma and those causing rickettsial disease, cause inflammation of the meninges and consequently encephalitis. Lyme disease or Bartonella henselae may also cause encephalitis.

Other infectious causes

Certain parasitic or protozoal infestations, such as toxoplasmosis and malaria can also cause encephalitis in people with compromised immune systems.

The rare but typically deadly forms of encephalitis, primary amoebic meningoencephalitis and Granulomatous amoebic encephalitis, are caused by free-living amoeba.

Autoimmune encephalitis

Autoimmune encephalitis signs can include catatonia, psychosis, abnormal movements, and autonomic dysregulation. Antibody-mediated anti-N-methyl-D-aspartate-receptor encephalitis and Rasmussen encephalitis are examples of autoimmune encephalitis.

Anti-NMDA receptor encephalitis is the most common autoimmune form, and is accompanied by ovarian teratoma in 58 percent of affected women 18–45 years of age.

Another autoimmune cause includes acute disseminated encephalitis, a demyelinating disease which primarily affects children.

Diagnosis

Spinal tap on a newborn

People should only be diagnosed with encephalitis if they have a decreased or altered level of consciousness, lethargy, or personality change for at least twenty-four hours without any other explainable cause. Diagnosing encephalitis is done via a variety of tests:

  • Brain scan, done by MRI, can determine inflammation and differentiate from other possible causes.
  • EEG, in monitoring brain activity, encephalitis will produce abnormal signal.
  • Lumbar puncture (spinal tap), this helps determine via a test using the cerebral-spinal fluid, obtained from the lumbar region.
  • Blood test
  • Urine analysis
  • Polymerase chain reaction (PCR) testing of the cerebrospinal fluid, to detect the presence of viral DNA which is a sign of viral encephalitis.

Prevention

Vaccination is available against tick-borne and Japanese encephalitis and should be considered for at-risk individuals. Post-infectious encephalomyelitis complicating smallpox vaccination is avoidable, for all intents and purposes, as smallpox is nearly eradicated. Contraindication to Pertussis immunization should be observed in patients with encephalitis.

Treatment

An ideal drug to treat brain infection should be small, moderately lipophilic at pH of 7.4, low level of plasma protein binding, volume of distribution of litre per kg, does not have strong affinity towards binding with P-glycoprotein, or other efflux pumps on the surface of blood–brain barrier. Some drugs such as isoniazid, pyrazinamide, linezolid, metronidazole, fluconazole, and some fluoroquinolones have good penetration to blood brain barrier. Treatment (which is based on supportive care) is as follows:

Pyrimethamine-based maintenance therapy is often used to treat toxoplasmic encephalitis (TE), which is caused by Toxoplasma gondii and can be life-threatening for people with weak immune systems. The use of highly active antiretroviral therapy (HAART), in conjunction with the established pyrimethamine-based maintenance therapy, decreases the chance of relapse in patients with HIV and TE from approximately 18% to 11%. This is a significant difference as relapse may impact the severity and prognosis of disease and result in an increase in healthcare expenditure.

The effectiveness of intravenous immunoglobulin for the management of childhood encephalitis is unclear. Systematic reviews have been unable to draw firm conclusions because of a lack of randomised double-blind studies with sufficient numbers of patients and sufficient follow-up. There is the possibility of a benefit of intravenous immunoglobulin for some forms of childhood encephalitis on some indicators such as length of hospital stay, time to stop spasms, time to regain consciousness, and time to resolution of neuropathic symptoms and fever. Intravenous immunoglobulin for Japanese encephalitis appeared to have no benefit when compared with placebo (pretend) treatment.

Prognosis

Identification of poor prognostic factors include cerebral edema, status epilepticus, and thrombocytopenia. In contrast, a normal encephalogram at the early stages of diagnosis is associated with high rates of survival.

Epidemiology

Encephalitis deaths per million persons in 2012
  0-0
  1-1
  2-2
  3-4
  5-9
  10-14
  15-24
  25-45

The number of new cases a year of acute encephalitis in Western countries is 7.4 cases per 100,000 people per year. In tropical countries, the incidence is 6.34 per 100,000 people per year. The number of cases of encephalitis has not changed much over time, with about 250,000 cases a year from 2005 to 2015 in the US. Approximately seven per 100,000 people were hospitalized for encephalitis in the US during this time. In 2015, encephalitis was estimated to have affected 4.3 million people and resulted in 150,000 deaths worldwide. Herpes simplex encephalitis has an incidence of 2–4 per million of the population per year.

Terminology

Encephalitis with meningitis is known as meningoencephalitis, while encephalitis with involvement of the spinal cord is known as encephalomyelitis.

The word is from Ancient Greek ἐγκέφαλος, enképhalos 'brain', composed of ἐν, en, 'in' and κεφαλή, kephalé, 'head', and the medical suffix -itis 'inflammation'.

Monday, February 17, 2025

F-test

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/F-test
An f-test pdf with d1 and d2 = 10, at a significance level of 0.05. (Red shaded region indicates the critical region)

An F-test is a statistical test that compares variances. It's used to determine if the variances of two samples, or if the ratios of variances among multiple samples, are significantly different. The test calculates a statistic, represented by the random variable F, and checks if it follows an F-distribution. This check is valid if the null hypothesis is true and standard assumptions about the errors (ε) in the data hold.

F-tests are frequently used to compare different statistical models and find the one that best describes the population the data came from. When models are created using the least squares method, the resulting F-tests are often called "exact" F-tests. The F-statistic was developed by Ronald Fisher in the 1920s as the variance ratio and was later named in his honor by George W. Snedecor.

Common examples

Common examples of the use of F-tests include the study of the following cases

F-test of the equality of two variances

The F-test is sensitive to non-normality. In the analysis of variance (ANOVA), alternative tests include Levene's test, Bartlett's test, and the Brown–Forsythe test. However, when any of these tests are conducted to test the underlying assumption of homoscedasticity (i.e. homogeneity of variance), as a preliminary step to testing for mean effects, there is an increase in the experiment-wise Type I error rate.

Formula and calculation

Most F-tests arise by considering a decomposition of the variability in a collection of data in terms of sums of squares. The test statistic in an F-test is the ratio of two scaled sums of squares reflecting different sources of variability. These sums of squares are constructed so that the statistic tends to be greater when the null hypothesis is not true. In order for the statistic to follow the F-distribution under the null hypothesis, the sums of squares should be statistically independent, and each should follow a scaled χ²-distribution. The latter condition is guaranteed if the data values are independent and normally distributed with a common variance.

One-way analysis of variance

The formula for the one-way ANOVA F-test statistic is

or

The "explained variance", or "between-group variability" is

where denotes the sample mean in the i-th group, is the number of observations in the i-th group, denotes the overall mean of the data, and denotes the number of groups.

The "unexplained variance", or "within-group variability" is

where is the jth observation in the ith out of groups and is the overall sample size. This F-statistic follows the F-distribution with degrees of freedom and under the null hypothesis. The statistic will be large if the between-group variability is large relative to the within-group variability, which is unlikely to happen if the population means of the groups all have the same value.

F Table: Level 5% Critical values, containing degrees of freedoms for both denominator and numerator ranging from 1-20

The result of the F test can be determined by comparing calculated F value and critical F value with specific significance level (e.g. 5%). The F table serves as a reference guide containing critical F values for the distribution of the F-statistic under the assumption of a true null hypothesis. It is designed to help determine the threshold beyond which the F statistic is expected to exceed a controlled percentage of the time (e.g., 5%) when the null hypothesis is accurate. To locate the critical F value in the F table, one needs to utilize the respective degrees of freedom. This involves identifying the appropriate row and column in the F table that corresponds to the significance level being tested (e.g., 5%).

How to use critical F values:

If the F statistic < the critical F value

  • Fail to reject null hypothesis
  • Reject alternative hypothesis
  • There is no significant differences among sample averages
  • The observed differences among sample averages could be reasonably caused by random chance itself
  • The result is not statistically significant

If the F statistic > the critical F value

  • Accept alternative hypothesis
  • Reject null hypothesis
  • There is significant differences among sample averages
  • The observed differences among sample averages could not be reasonably caused by random chance itself
  • The result is statistically significant

Note that when there are only two groups for the one-way ANOVA F-test, where t is the Student's statistic.

Advantages

  • Multi-group Comparison Efficiency: Facilitating simultaneous comparison of multiple groups, enhancing efficiency particularly in situations involving more than two groups.
  • Clarity in Variance Comparison: Offering a straightforward interpretation of variance differences among groups, contributing to a clear understanding of the observed data patterns.
  • Versatility Across Disciplines: Demonstrating broad applicability across diverse fields, including social sciences, natural sciences, and engineering.

Disadvantages

  • Sensitivity to Assumptions: The F-test is highly sensitive to certain assumptions, such as homogeneity of variance and normality which can affect the accuracy of test results.
  • Limited Scope to Group Comparisons: The F-test is tailored for comparing variances between groups, making it less suitable for analyses beyond this specific scope.
  • Interpretation Challenges: The F-test does not pinpoint specific group pairs with distinct variances. Careful interpretation is necessary, and additional post hoc tests are often essential for a more detailed understanding of group-wise differences.

Multiple-comparison ANOVA problems

The F-test in one-way analysis of variance (ANOVA) is used to assess whether the expected values of a quantitative variable within several pre-defined groups differ from each other. For example, suppose that a medical trial compares four treatments. The ANOVA F-test can be used to assess whether any of the treatments are on average superior, or inferior, to the others versus the null hypothesis that all four treatments yield the same mean response. This is an example of an "omnibus" test, meaning that a single test is performed to detect any of several possible differences. Alternatively, we could carry out pairwise tests among the treatments (for instance, in the medical trial example with four treatments we could carry out six tests among pairs of treatments). The advantage of the ANOVA F-test is that we do not need to pre-specify which treatments are to be compared, and we do not need to adjust for making multiple comparisons. The disadvantage of the ANOVA F-test is that if we reject the null hypothesis, we do not know which treatments can be said to be significantly different from the others, nor, if the F-test is performed at level α, can we state that the treatment pair with the greatest mean difference is significantly different at level α.

Regression problems

Consider two models, 1 and 2, where model 1 is 'nested' within model 2. Model 1 is the restricted model, and model 2 is the unrestricted one. That is, model 1 has p1 parameters, and model 2 has p2 parameters, where p1 < p2, and for any choice of parameters in model 1, the same regression curve can be achieved by some choice of the parameters of model 2.

One common context in this regard is that of deciding whether a model fits the data significantly better than does a naive model, in which the only explanatory term is the intercept term, so that all predicted values for the dependent variable are set equal to that variable's sample mean. The naive model is the restricted model, since the coefficients of all potential explanatory variables are restricted to equal zero.

Another common context is deciding whether there is a structural break in the data: here the restricted model uses all data in one regression, while the unrestricted model uses separate regressions for two different subsets of the data. This use of the F-test is known as the Chow test.

The model with more parameters will always be able to fit the data at least as well as the model with fewer parameters. Thus typically model 2 will give a better (i.e. lower error) fit to the data than model 1. But one often wants to determine whether model 2 gives a significantly better fit to the data. One approach to this problem is to use an F-test.

If there are n data points to estimate parameters of both models from, then one can calculate the F statistic, given by

where RSSi is the residual sum of squares of model i. If the regression model has been calculated with weights, then replace RSSi with χ2, the weighted sum of squared residuals. Under the null hypothesis that model 2 does not provide a significantly better fit than model 1, F will have an F distribution, with (p2p1np2) degrees of freedom. The null hypothesis is rejected if the F calculated from the data is greater than the critical value of the F-distribution for some desired false-rejection probability (e.g. 0.05). Since F is a monotone function of the likelihood ratio statistic, the F-test is a likelihood ratio test.

Knockout mouse

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