For a recent discussion of problems with random sampling plus some solutions, go to David Morgan’s comments at:

www.methodspace.com/forums/topic/two-problems-with-random-sampling-and-two-alternatives

Seawright and Gerring (2008) provide a useful discussion of purposive sampling in political science research. Go to:

http://prq.sagepub.com/content/61/2/294.full.pdf+html