Why You Only Need to Test With Five Users
http://www.useit.com/alertbox/20000319.html

Nielsen, Jakob
Alertbox
2000

Abstract:

Some people think that usability is very costly and complex and that user tests should be reserved for the rare web design project with a huge budget and a lavish time schedule. Not true. Elaborate usability tests are a waste of resources. The best results come from testing no more than 5 users and running as many small tests as you can afford.