CSC/ECE 517 Fall 2007/wiki2 9 A3

Principle of Least Astonishment

The Principle of Least Astonishment states that the result of performing some operation should be obvious, consistent, and predictable, based upon the name of the operation and other clues. It is a general principle in the design of all kinds of interfaces, not just software: “Do the least surprising thing”. It's a consequence of the fact that human beings can only pay attention to one thing at one time. Surprises in the interface focus that single locus of attention on the interface, rather than on the task where it belongs. The principle of least astonishment should not be interpreted as a call for mechanical conservatism in design. Novelty raises the cost of a user's first few interactions with an interface, but poor design will make the interface needlessly painful forever. As in other sorts of design, rules are not a substitute for good taste and engineering judgment.

Example

The name of a function should reflect what it does. Otherwise, a user of the function will be unpleasantly surprised: [2]

Bad:

int multiply(int a, int b)
{
  return a + b;
}

Better:

int multiply(int a, int b)
{
  return a * b;
}

Or:

int add(int a, int b)
{
  return a + b;
}

References

[1] http://www.faqs.org/docs/artu/ch11s01.html
[2] http://c2.com/cgi/wiki?PrincipleOfLeastAstonishment