A qualitative approach to selecting who to interview or where to conduct site visits by selecting information-rich cases for in-depth study. Information-rich cases are those from which one can learn a great deal about issues of central importance to the purpose of the evaluation. Examples are studying success cases, failures and drop-outs, or examples of major public importance.
Patton, M. Q. (2015). Qualitative research and evaluation methods, 4th ed. Thousand Oaks, CA: Sage.