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A case study focuses on a particular unit - a person, a site, a project. It often uses a combination of quantitative and qualitative data. Case studies can be particularly useful for understanding how different elements fit together and how different elements (implementation, context and other factors) have produced the observed impacts. There are different types of case studies, which can be used for different purposes in evaluation. The GAO (Government Accountability Office) has described six different types of case study: 1. Illustrative: This is descriptive in character and intended to add realism and in-depth examples to other information about a program or policy. (These are often used to complement quantitative data by providing examples of the overall findings). 2. Exploratory: This is also descriptive but is aimed at generating hypotheses for later investigation rather than simply providing illustration. 3. Critical instance: This examines a single instance of unique interest, or serves as a critical test of an assertion about a program, problem or strategy. 4. Program implementation. This investigates operations, often at several sites, and often with reference to a set of norms or standards about implementation processes. 5. Program effects. This examines the causal links between the program and observed effects (outputs, outcomes or impacts, depending on the timing of the evaluation) and usually involves multisite, multimethod evaluations. 6. Cumulative. This brings together findings from many case studies to answer evaluative questions.