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A case study is a report about a person, group, or situation that has not been studied. If the case study, for instance, is about a group, it describes the behavior of the group as the behavior of each individual, not the behavior of whole in the group.
Case studies can be produced by following a informal 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. Case studies may involve both qualitative and quantitative research methods.
Case studies in research may be mistaken for the case method used in teaching.
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.
Four types of cases may thus be distinguished for selection:
Whatever the frame of reference for the choice of the subject of the case study (key, outlier, local knowledge), there is a distinction to be made between the subjestorical unity[page needed] through which the theoretical focus of the study is being viewed. The object is that theoretical focus - the analytical frame. Thus, for example, if a researcher were interested in US resistance to communist expansion as a theoretical focus, then the Korean War might be taken to be the subject, the lens, the case study through which the theoretical focus, the object, could be viewed and explicated.
Beyond decisions about case selection and the subject and object of the study, decisions need to be made about purpose, approach and process in the case study. Thomas thus proposes a typology for the case study wherein purposes are first identified (evaluative or exploratory), then approaches are delineated (theory-testing, theory-building or illustrative), then processes are decided upon, with a principal choice being between whether the study is to be single or multiple, and choices also about whether the study is to be retrospective, snapshot or diachronic, and whether it is nested, parallel or sequential. It is thus possible to take many routes through this typology, with, for example, an exploratory, theory-building, multiple, nested study, or an evaluative, theory-testing, single, retrospective study. The typology thus offers many permutations for case-study structure.
A closely related study in medicine is the case report, which identifies a specific case as treated and/or examined by the authors as presented in a novel form. These are, to a differentiable degree, similar to the case study in that many contain reviews of the relevant literature of the topic discussed in the thorough examination of an array of cases published to fit the criterion of the report being presented. These case reports can be thought of as brief case studies with a principal discussion of the new, presented case at hand that presents a novel interest.
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Under the more generalized category of case study exists several subdivisions, each of which is custom selected for use depending upon the goals and/or objectives of the investigator. These types of case study include the following:
Case Studies in Business
At Harvard Law School In 1870, Christopher Columbus Langdell departed from the traditional lecture-and-notes approach to teaching contract law and began using cases pled before courts as the basis for class discussions . By 1920, this practice had become the dominant pedagogical approach used by law schools in the United States ; it also was adopted by Harvard Business School.
Research in business disciplines is usually based on a positivist epistemology,  namely, that reality is something that is objective and can be discovered and understood by a scientific examination of empirical evidence. But organizational behavior cannot always be easily reduced to simple tests that prove something to be true or false. Reality may be an objective thing, but it is understood and interpreted by people who, in turn, act upon it, and so critical realism  which addresses the connection between the natural and social worlds is a useful basis for analyzing the environment of and events within an organization.
Case studies in management are generally used to interpret strategies or relationships, to develop sets of "best practices", or to analyze the external influences or the internal interactions of a firm.  With several notable exceptions (i.e., Janis on Groupthink ), they are rarely used to propose new theories.
A critical case is defined as having strategic importance in relation to the general problem. A critical case allows the following type of generalization: "If it is valid for this case, it is valid for all (or many) cases." In its negative form, the generalization would run: "If it is not valid for this case, then it is not valid for any (or valid for only few) cases."
The case study is effective for generalizing using the type of test that Karl Popper called falsification, which forms part of critical reflexivity. Falsification offers one of the most rigorous tests to which a scientific proposition can be subjected: if just one observation does not fit with the proposition it is considered not valid generally and must therefore be either revised or rejected. Popper himself used the now famous example: "All swans are white", and proposed that just one observation of a single black swan would falsify this proposition and in this way have general significance and stimulate further investigations and theory-building. The case study is well suited for identifying "black swans" because of its in-depth approach: what appears to be "white" often turns out on closer examination to be "black".
Galileo Galilei built his rejection of Aristotle's law of gravity on a case study selected by information-oriented sampling and not by random sampling. The rejection consisted primarily of a conceptual experiment and later on of a practical one. These experiments, with the benefit of hindsight, are self-evident. Nevertheless, Aristotle's incorrect view of gravity had dominated scientific inquiry for nearly two thousand years before it was falsified. In his experimental thinking, Galileo reasoned as follows: if two objects with the same weight are released from the same height at the same time, they will hit the ground simultaneously, having fallen at the same speed. If the two objects are then stuck together into one, this object will have double the weight and will according to the Aristotelian view therefore fall faster than the two individual objects. This conclusion seemed contradictory to Galileo.
Other roots stem from the early 20th century, when researchers working in the disciplines of sociology, psychology, and anthropology began making case studies. In all these disciplines, case studies were an occasion for postulating new theories, as in the grounded-theory work of sociologists Barney Glaser (1930- ) and Anselm Strauss (1916-1996).
The popularity of case studies in testing theories or hypotheses has developed only in recent decades. One of the areas in which case studies have been gaining popularity is education and in particular educational evaluation.
Educators have used case studies as a teaching method and as part of professional development, especially in business and legal education. The problem-based learning (PBL) movement offers an example. When used in (non-business) education and professional development, case studies are often referred to[by whom?] as critical incidents.
Ethnography exemplifies a type of case study, commonly found in communication case studies. Ethnography is the description, interpretation, and analysis of a culture or social group, through field research in the natural environment of the group being studied. The main method of ethnographic research is thorough observation, where the researcher observes study participants over an extended period of time within the participants' own environment.
Comparative case studies have become more popular[when?] in social science, policy, and education research. One approach encourages researchers to compare horizontally, vertically, and temporally.
Using case studies in research differs from their use in teaching, where they are commonly called case methods and casebook methods. Teaching case studies have been a highly popular pedagogical format in many fields ranging from business education to science education. Harvard Business School has possibly been a developer and user of teaching case studies. Business school faculty generally develop case studies with particular learning objectives in mind. Additional relevant documentation, such as financial statements, time-lines, and short biographies, often referred to in the case study as exhibits, and multimedia supplements (such as video-recordings of interviews with the case subject) often accompany the case studies. Similarly, teaching case studies have become increasingly popular in science education. The National Center for Case Studies in Teaching Science has made a growing body of case studies available for classroom use, for university as well as secondary school coursework. Nevertheless, the principles in doing case study research contrast with those in doing case studies for teaching. Teaching case studies need not adhere strictly to the use of evidence, as they can be manipulated to satisfy educational needs. The generalizations from teaching case studies also may relate to pedagogical issues rather than the substance of the case being studied.
Case studies are commonly used in case competitions and interviews for consulting firms such as McKinsey & Company, CEB Inc. and the Boston Consulting Group, in which candidates are asked to develop the best solution for a case in an allotted time frame.
There are three major types of case studies common to public relations: linear, process-oriented and grounded.