A data collection technique involving the examination of a limited number of specific cases or projects which the evaluator anticipates will be revealing about a program or policy as a whole.
(UK Evaluation Society Glossary of Evaluation Terms. http://www.evaluation.org.uk/resources/glossary.aspx.)
The following is drawn from the UK Evaluation Society's Glossary of Evaluation Terms:
"Case studies tend to be appropriate where it is extremely difficult to choose a sample large enough to be statistically generalisable to the population as a whole; where generalization is not important; where in-depth, usually descriptive data is required; and where the cases or projects to be studied are likely to be quite complex. See also case study designs, data collection."