My take is that it’s a very complicated issue. When Bob Virzi and others conducted research in the 1980s on the number of users, it was found that five to eight users was typically enough for testing. But the field has changed since the research was conducted.

Since these initial studies, there are many differences. Many more people are using the same application for different task goals. Also, the audiences today are much larger than they were in the 1980s. Interfaces are just much more complex because many of the UIs we use are embedded in other things.

For example, how do you conduct a usability test with only 5-8 people on an iPhone with 25 applications on it? We’re talking about a highly customizable mobile device where the individual experience is very different from one person to the next. This wasn’t true in the early studies on the number of users.

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— Perfetti Media » The Evolution of Usability Tests: An Interview with Dana Chisnell