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 but also case studies of clinical practices.[3][4][5] However, when case is used in an abstract sense, as in a claim, proposition, or argument, such a case can be the subject of many research methods, not just a case study.
Thomas[6] 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."[7]