Robert Kosara and Christopher G. Healey and Victoria Interrante and David H. Laidlaw and Colin Ware.
User Studies: Why, How, and When?.
In IEEE Computer Graphics and Applications, vol. 23, no. 4, pp. 20--25, 2003.

Bibtex:

@Article{ 	kosara:2003:USWH,
  author = 	"Robert Kosara and Christopher G. Healey and Victoria
                  Interrante and David H. Laidlaw and Colin Ware",
  title = 	"User Studies: Why, How, and When?",
  journal = 	"IEEE Computer Graphics and Applications",
  volume = 	"23",
  number = 	"4",
  pages = 	"20--25",
  month = 	"July/August",
  year = 	"2003",
}