5-user assumption

research paper by Laura Faulkner of the University of Texas, Austin on the benefits of usability testing with more users - Beyond the five-user assumption: Benefits of increased sample sizes in usability testing (pdf)

essentially the author says that people like Jakob Neilsen who had stated that testing with 5 users is enough did so at a time when usability testing was not widely embraced or funded in project. Now that usability testing is more frequently a major component of development, the number of users needs to increase in order to increase the number of usability problems found.

The more powerful argument for implementing software usability testing, then, is not that it can be done cheaply with, say, 5 test users, but that the implications of missing usability problems are severe enough to warrant investment in fully valid test practices.