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Figure 1 Illustration showing which evaluators found which usability problems in a heuristic evaluation of a banking system.

My recommendation is normally to use three to five evaluators since one does not gain that much additional information by using larger numbers. Therefore, it is necessary to involve multiple evaluators in any heuristic evaluation (see below for a discussion of the best number of evaluators).

Second, some of the hardest-to-find usability problems (represented by the leftmost columns in Figure 1) are found by evaluators who do not otherwise find many usability problems. First, it is not necessarily true that the same person will be the best evaluator every time. Furthermore, one cannot just identify the best evaluator and rely solely on that person's findings. It is certainly true that some usability problems are so easy to find that they are found by almost everybody, but there are also some problems that are found by very few evaluators.

The figure clearly shows that there is a substantial amount of nonoverlap between the sets of usability problems found by different evaluators. Each of the black squares in Figure 1 indicates the finding of one of the usability problems by one of the evaluators. Figure 1 shows an example from a case study of heuristic evaluation where 19 evaluators were used to find 16 usability problems in a voice response system allowing customers access to their bank accounts (Nielsen 1992). Therefore, it is possible to improve the effectiveness of the method significantly by involving multiple evaluators. Luckily, experience from many different projects has shown that different people find different usability problems. In general, heuristic evaluation is difficult for a single individual to do because one person will never be able to find all the usability problems in an interface. Heuristic evaluation involves having a small set of evaluators examine the interface and judge its compliance with recognized usability principles (the "heuristics"). Heuristic evaluation (Nielsen and Molich, 1990 Nielsen 1994) is a usability engineering method for finding the usability problems in a user interface design so that they can be attended to as part of an iterative design process.