There are two ways to learn how to build a house. One might study the construction of many houses – perhaps a large subdivision or even hundreds of thousands of houses. Or one might study the construction of a particular house. The first approach is a cross-case method. The second is a within-case or case study method. While both are concerned with the same general subject – the building of houses – they follow different paths to this goal.

The same could be said about social research. Researchers may choose to observe lots of cases superficially, or a few cases more intensively. (They may of course do both, as recommended in this book. But there are usually trade-offs involved in this methodological choice.)

For anthropologists and sociologists, the key unit is often the social group (family, ethnic group, village, religious group, etc.). For psychologists, it is usually the individual. For economists, it may be the individual, the firm, or some larger agglomeration. For political scientists, the topic is often nation-states, regions, organizations, statutes, or elections.

In all these instances, the case study – of an individual, group, organization or event – rests implicitly on the existence of a micro-macro link in social behavior. It is a form of cross-level inference. Sometimes, in-depth knowledge of an individual example is more helpful than fleeting knowledge about a larger number of examples. We gain better understanding of the whole by focusing on a key part.