Principle of least astonishment

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.^{[citation needed]}

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 she has just typed her password into her IM client and sent it to her 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. In order 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 Ctrl+Q 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 Ctrl+Q as part (most likely the last part) of the macro. The principle says that pressing Ctrl+Q while recording a macro should not quit the program (which would surprise the user), but rather should record the keystroke.

See also

• DWIM (Do What I Mean)
• Occam's razor
• Law of Demeter (also known as the "principle of least knowledge")

References

1. ^ Joshua Bloch (2006), How to design a good API and why it matters, Association for Computing Machinery, pp. 506-507, http://portal.acm.org/citation.cfm?id=1176622 

External links

• "Applying the Rule of Least Surprise" from The Art of Unix Programming by E.S. Raymond
• Principle of Least Astonishment at Portland Pattern Repository
• Principle of least astonishment on UXPassion.com
• Law of Least Astonishment from The Tao of Programming by Geoffrey James