Focus Groups vs. Usability Testing: What, When and Why?
http://www.webcredible.co.uk/user-friendly-resources/web-usability/focus-vs-testing.shtml

Gray, Alistair
Webcredible
2009

Abstract:

Focus groups and usability testing are two very useful but very different user research disciplines. This article will look at the difference between focus groups and usability testing, the pros and cons of each and when in the development process you should use them.