Principle of least astonishment
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The principle of least astonishment (POLA/PLA) applies to user interface design, software design, and ergonomics. It is alternatively referred to as the rule or law of least astonishment, or the rule or principle of least surprise (POLS).
The POLA states that, when two elements of an interface conflict, or are ambiguous, the behaviour 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.[1]
This practice also involves the application of sensible defaults.
Examples
- A user is about to enter his username and password for a program or website when he receives an instant message. Some instant messaging clients will immediately grab the keyboard focus and move it into their own response field, because they assume the user will want to respond to the new message immediately. In reality, the user may be astonished to find that they have just typed their password into their IM client and sent it to their friends. This conflict arises because the two programs are not aware of each other's existence, and cannot easily determine when they might get in each other's way. To avoid such conflicts, operating systems may restrict the interaction of different programs, for example by preventing the IM client from stealing the focus.
- A user interface may have the behaviour that pressing <kbd class="keyboard-key" style="border: 1px solid; border-color: #ddd #bbb #bbb #ddd; border-bottom-width: 2px; -moz-border-radius: 3px; -webkit-border-radius: 3px; border-radius: 3px; background-color: #f9f9f9; padding: 1px 3px; font-family: inherit; font-size: 0.85em; white-space: nowrap;">Ctrl</kbd>+<kbd class="keyboard-key" style="border: 1px solid; border-color: #ddd #bbb #bbb #ddd; border-bottom-width: 2px; -moz-border-radius: 3px; -webkit-border-radius: 3px; border-radius: 3px; background-color: #f9f9f9; padding: 1px 3px; font-family: inherit; font-size: 0.85em; white-space: nowrap;">Q</kbd> causes the program to quit. The same user interface may have a facility for recording macros, a sequence of keystrokes to be played back later, intended to be able to control all aspects of the program. The user may want to record a keystroke sequence that includes <kbd class="keyboard-key" style="border: 1px solid; border-color: #ddd #bbb #bbb #ddd; border-bottom-width: 2px; -moz-border-radius: 3px; -webkit-border-radius: 3px; border-radius: 3px; background-color: #f9f9f9; padding: 1px 3px; font-family: inherit; font-size: 0.85em; white-space: nowrap;">Ctrl</kbd>+<kbd class="keyboard-key" style="border: 1px solid; border-color: #ddd #bbb #bbb #ddd; border-bottom-width: 2px; -moz-border-radius: 3px; -webkit-border-radius: 3px; border-radius: 3px; background-color: #f9f9f9; padding: 1px 3px; font-family: inherit; font-size: 0.85em; white-space: nowrap;">Q</kbd> as part (most likely the last part) of the macro. The principle says that pressing <kbd class="keyboard-key" style="border: 1px solid; border-color: #ddd #bbb #bbb #ddd; border-bottom-width: 2px; -moz-border-radius: 3px; -webkit-border-radius: 3px; border-radius: 3px; background-color: #f9f9f9; padding: 1px 3px; font-family: inherit; font-size: 0.85em; white-space: nowrap;">Ctrl</kbd>+<kbd class="keyboard-key" style="border: 1px solid; border-color: #ddd #bbb #bbb #ddd; border-bottom-width: 2px; -moz-border-radius: 3px; -webkit-border-radius: 3px; border-radius: 3px; background-color: #f9f9f9; padding: 1px 3px; font-family: inherit; font-size: 0.85em; white-space: nowrap;">Q</kbd> while recording a macro should not quit the program (which would surprise the user), but rather should record the keystroke.
See also
References
External links