Usability Myths Need Reality Checks
http://www.uie.com/articles/usability_myths/

Schroeder, Will
User Interface Engineering
2002

Abstract:

Not so very long ago, it was agreed that five to eight users was enough for a good usability test. Somehow, this idea achieved mythic status. We believed it. We preached it to everyone who would listen. It survived in areas where it had been disproved, and was introduced into new situations where it didn't even apply. What gives some ideas such staying power? What did the five-user myth accomplish? It reconciled test plans with testing budgets! If five to eight users are enough, then it's safe to act on the results of a test series with only five to eight users.