Topics in Usability
How Many Users?

Articles

One of the great debates in usability is "How many users do you need to find most usabilty problems?"

Articles

In What's in a Number (Usability Interface, January 2003), Carol Barnum reviews the research

How many users does it take for a (web) usability test? - Idea Market 2004, led by Carol Barnum

In How Many Users Does it Take to Change a Web Site? William Hudson compares two points of view.

According to this paper, the first 3 users identify 68% of issues. Nielsen, J., and Landuaer, T. K. (1993). A mathematical model of the finding of usability problems. In Proceedings of the ACM INTERCHI'93 Conference Amsterdam, The Netherlands, April 24-29, pp. 206-213). ACM, New York, NY.

The mathematical models for Nielsen and Landauer are from: Virzi, R (1992) Refing the test phase of usability evaluation: How many subjects is enough? Human Factors, 34, 457-468.

Jared Spool's paper at CHI2000 said that they were seeing 5 new obstacles with each test, with only 3.2% repeated, and that it would take 90 or more users to identify 85% of the obstacles

Another point of view is that all you need to do is find enough problems to fix to warrant another iteration of the design.