Yin suggests that case-study designs vary according to two distinct dimensions. One dimension accounts for the number of "cases" being studied: the presence of either single- or multiple-case designs. A second dimension allows for case studies to be either "holistic" (studying the entire unit of analysis as a single global entity) or "embedded" (allowing multiple units of analysis to be studied for the purpose of understanding their interworkings). According to Yin, this classification system leaves the researcher with a choice among four different design types: single-case (holistic) design, single-case (embedded) design, multiple-case (holistic) design, and multiple-case (embedded) design. Choosing among these designs involves the kinds of research questions that the researcher is attempting to answer.
Case-study methods are initiated for a variety of reasons, one of which is to serve as a vehicle for exploratory research. As a new research area begins to develop, the initial uncharted territory is sometimes best studied (particularly when the research questions are ill-defined) using a case-study method to determine which direction should be pursued first. This method has therefore been commonly misperceived as being able to contribute only in a limited exploratory capacity; however, the case study can, and should, be used not only to help focus initial research questions but also to describe and explain behaviors. As Yin makes clear, both "how" questions and "why" questions can be answered by this approach.
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