The Use and Misuse of Focus Groups
http://www.useit.com/papers/focusgroups.html

Nielsen, Jakob
Useit
1997

Abstract:

Focus groups often bring out users' spontaneous reactions and ideas and let you observe some group dynamics and organizational issues. You can also ask people to discuss how they perform activities that span many days or weeks: something that is expensive to observe directly. However, they can only assess what customers say they do and not the way customers actually operate the product. Since there are often major differences between what people say and what they do, direct observation of one user at a time always needs to be done to supplement focus groups.