Determine the Number of Participants to Test

The number of participants you decide to test depends on many factors, including:

■ The degree of confidence in the results that you require

■ The number of available resources to set up and conduct the test

■ The availability of the type of participants you require

■ The duration of the test session

Ultimately, you have to balance your need for acquiring participants with these (and probably other) practical constraints of time and resources. If you require statistically valid results, you will need to test enough participants to conduct the appropriate analyses and generalize to your specific target population, as well as to rigorously control for potentially biasing conditions and factors.

If, however, you are simply attempting to expose as many usability problems as possible in the shortest amount of time, then test at least four to five participants of each user or audience cell. Research indicates that testing four to five participants of each type or cell will expose the vast majority of usability problems. However, we add that we are a bit uncomfortable testing only four participants, and we try to test at least eight participants if possible. While you may expose most of the usability problems with four participants, there is still a good chance you may overlook a problem that could have severe ramifications.

An important consideration is whether you will conduct more than one test during the product development life cycle. 1/ you are going to conduct multiple tests, then you can feel more confident testing fewer participants in each test. For example, if you conduct three tests with five participants each, you end up testing fifteen people. If you will be conducting a single test, however, then you will be better served by having more participants for that lone test.