A case study is a "published report about a person, group, or situation that has been studied over time. If the case study is about a group, it describes the behavior of the group as a whole, not behavior of each individual in the group. Case studies can be produced by following a formal research method. These case studies are likely to appear in formal research venues, as journals and professional conferences, rather than popular works. The resulting body of 'case study research' has long had a prominent place in many disciplines and professions, ranging from psychology, anthropology, sociology, and political science to education, clinical science, social work, and administrative science.
In doing case study research, the "case" being studied may be an individual, organization, event, or action, existing in a specific time and place. For instance, clinical science has produced both well-known case studies of individuals and also case studies of clinical practices. However, when "case" is used in an abstract sense, as in a claim, a proposition, or an argument, such a case can be the subject of many research methods, not just case study research.
Thomas offers the following definition of case study:
"Case studies are analyses of persons, events, decisions, periods, projects, policies, institutions, or other systems that are studied holistically by one or more method. The case that is the subject of the inquiry will be an instance of a class of phenomena that provides an analytical frame — an object — within which the study is conducted and which the case illuminates and explicates."
According to J. Creswell, data collection in a case study occurs over a "sustained period of time."
One approach sees the case study defined as a research strategy, an empirical inquiry that investigates a phenomenon within its real-life context. Case-study research can mean single and multiple case studies, can include quantitative evidence, relies on multiple sources of evidence, and benefits from the prior development of theoretical propositions. As such, case study research should not be confused with qualitative research, as case studies can be based on any mix of quantitative and qualitative data. Similarly, single-subject research might be taken as case studies of a sort, except that the repeated trials in single-subject research permit the use of experimental designs that would not be possible in typical case studies. At the same time, the repeated trials can provide a statistical framework for making inferences from quantitative data.
The case study is sometimes mistaken for the case method used in teaching, but the two are not the same.
Case selection and structure
An average, or typical case, is often not the richest in information. In clarifying lines of history and causation it is more useful to select subjects that offer an interesting, unusual or particularly revealing set of circumstances. A case selection that is based on representativeness will seldom be able to produce these kinds of insights. When selecting a subject for a case study, researchers will therefore use information-oriented sampling, as opposed to random sampling. Outlier cases (that is, those which are extreme, deviant or atypical) reveal more information than the potentially representative case. Alternatively, a case may be selected as a key case, chosen because of the inherent interest of the case or the circumstances surrounding it. Alternatively it may be chosen because of a researchers' in-depth local knowledge; where researchers have this local knowledge they are in a position to “soak and poke” as Fenno puts it, and thereby to offer reasoned lines of explanation based on this rich knowledge of setting and circumstances.
Three types of cases may thus be distinguished:
1. Key cases
2. Outlier cases
3. Local knowledge cases