The principle of least astonishment (POLA), also called the principle of least surprise (alternatively a "law" or "rule") applies to user interface and software design.
A typical formulation of the principle, from 1984, is: "If a necessary
feature has a high astonishment factor, it may be necessary to redesign
the feature."
More generally, the principle means that a component of a system should behave in a way that most users will expect it to behave; the behavior should not astonish or surprise users.
More generally, the principle means that a component of a system should behave in a way that most users will expect it to behave; the behavior should not astonish or surprise users.
Formulation
A textbook formulation is: "People are part of the system. The design should match the user's experience, expectations, and mental models."
The principle aims to leverage the pre-existing knowledge of users to minimize the learning curve,
for instance by designing interfaces that borrow heavily from
"functionally similar or analogous programs with which your users are
likely to be familiar". User expectations in this respect may be closely related to a particular computing platform or tradition. For example, Unix command line programs are expected to follow certain conventions with respect to switches, and widgets of Microsoft Windows programs are expected to follow certain conventions with respect to keyboard shortcuts. In more abstract settings like an API, the expectation that function or method names intuitively match their behavior is another example. This practice also involves the application of sensible defaults.
When two elements of an interface conflict, or are ambiguous, the behavior should be that which will least surprise the user; in particular a programmer
should try to think of the behavior that will least surprise someone
who uses the program, rather than that behavior that is natural from
knowing the inner workings of the program.
The choice of "least surprising" behavior can depend on the expected audience (for example, end users, programmers, or system administrators).
Examples
A website could have an input field that focuses automatically after the page loads, such as a search field (e.g. Google Custom Search), or the username field of a login form.
Sites offering keyboard shortcuts often allow pressing ? to see the available shortcuts. Examples include Gmail and Jira.
In Windows operating systems and some desktop environments for Linux, the F1 function key typically opens the help program for an application. A similar keyboard shortcut in macOS is ⌘ Command+⇧ Shift+/. Users expect a help window or context menu
when they press the usual help shortcut key(s). Software that instead
uses this shortcut for another feature is likely to cause astonishment
if no help appears. Malware may exploit user familiarity with regular shortcuts.
A programming language's standard library usually provides a function similar to the pseudocode
ParseInteger(string, radix), which creates a machine-readable integer from a string of human-readable digits. The radix conventionally defaults to 10, meaning the string is interpreted as decimal (base 10). This function usually supports other bases, like binary (base 2) and octal (base 8), but only when they are specified explicitly. In a departure from this convention, JavaScript originally defaulted to base 8 for strings beginning with "0", causing developer confusion and software bugs. This was discouraged in ECMAScript 3 and dropped in ECMAScript 5.
