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In the social sciences and life sciences, a case study (or case report) is a descriptive or explanatory analysis of a person, group or event. An explanatory case study is used to explore causation in order to find underlying principles. Case studies may be prospective (in which criteria are established and cases fitting the criteria are included as they become available) or retrospective (in which criteria are established for selecting cases from historical records for inclusion in the study).
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 methods. 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."
Another suggestion is that case study should be 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. Case studies should not be confused with qualitative research and they can be based on any mix of quantitative and qualitative evidence. Single-subject research provides the statistical framework for making inferences from quantitative case-study data. This is also supported and well-formulated in (Lamnek, 2005): "The case study is a research approach, situated between concrete data taking techniques and methodologic paradigms."
The case study is sometimes mistaken for the case method, 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 potentialy 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. Or it may be chosen because of 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:
- Key cases
- Outlier cases
- Local knowledge cases
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”  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.
Generalizing from case studies 
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 be, ‘If it is not valid for this case, then it is not valid for any (or only few) cases.’
The case study is also effective for generalizing using the type of test that Karl Popper called falsification, which forms part of critical reflexivity. Falsification is 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 of, "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’s rejection of Aristotle’s law of gravity was based on a case study selected by information-oriented sampling and not 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 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. The only way to avoid the contradiction was to eliminate weight as a determinant factor for acceleration in free f
History of the case study 
It is generally believed that the case-study method was first introduced into social science by Frederic Le Play in 1829 as a handmaiden to statistics in his studies of family budgets. (Les Ouvriers Europeens (2nd edition, 1879).
The use of case studies for the creation of new theory in social sciences has been further developed by the sociologists Barney Glaser and Anselm Strauss who presented their research method, Grounded theory, in 1967.
The popularity of case studies in testing 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.
Case studies have also been used as a teaching method and as part of professional development, especially in business and legal education. The problem-based learning (PBL) movement is such an example. When used in (non-business) education and professional development, case studies are often referred to as critical incidents.
When the Harvard Business School was started, the faculty quickly realized that there were no textbooks suitable to a graduate program in business. Their first solution to this problem was to interview leading practitioners of business and to write detailed accounts of what these managers were doing. Cases are generally written by business school faculty with particular learning objectives in mind and are refined in the classroom before publication. Additional relevant documentation (such as financial statements, time-lines, and short biographies, often referred to in the case as "exhibits"), multimedia supplements (such as video-recordings of interviews with the case protagonist), and a carefully crafted teaching note often accompany cases.hahah
See also 
- Shepard, Jon; Robert W. Greene (2003). Sociology and You. Ohio: Glencoe McGraw-Hill. pp. A–22. ISBN 0-07-828576-3.
- Robert K. Yin. Case Study Research: Design and Methods. Fourth Edition. SAGE Publications. California, 2009. ISBN 978-1-4129-6099-1
- G. Thomas (2011) A typology for the case study in social science following a review of definition, discourse and structure. Qualitative Inquiry, 17, 6, 511-521
- Siegfried Lamnek. Qualitative Sozialforschung. Lehrbuch. 4. Auflage. Beltz Verlag. Weihnhein, Basel, 2005
- R. Fenno (1986) Observation, context, and sequence in the study of politics. American Political Science Review, 80, 1, 3-15
- M. Wieviorka (1992) Case studies: history or sociology? In C.C. Ragin and H.S. Becker (Eds) What is a case? Exploring the foundations of social inquiry. New York: Cambridge University Press.
- Gary Thomas, How to do your Case Study (Thousand Oaks: Sage, 2011)
- Sister Mary Edward Healy, C. S. J. (1947). "Le Play's Contribution to Sociology: His Method". The American Catholic Sociological Review 8 (2): 97–110. doi:10.2307/3707549.
- Robert E. Stake, The Art of Case Study Research (Thousand Oaks: Sage, 1995). ISBN 0-8039-5767-X
Useful Sources 
- Baxter, P and Jack, S. (2008) "Qualitative Case Study Methodology: Study design and implementation for novice researchers", in The Qualitative Report, 13(4): 544-559. Available from 
- Dul, J. and Hak, T. (2008) Case Study Methodology in Business Research. Oxford: Butterworth-Heinemann. ISBN 978-0-7506-8196-4.
- Eisenhardt, K. M. (1989) "Building theories from case study research", in The Academy of Management Review, 14 (4), Oct, 532-550. doi:10.2307/258557
- George, Alexander L. and Bennett, Andrew. (2005) Case studies and theory development in the social sciences. London: MIT Press. ISBN 0-262-57222-2
- Gerring, John. (2005) Case Study Research. New York: Cambridge University Press. ISBN 978-0-521-67656-4
- Hancké, Bob. (2009) Intelligent Research Design. A guide for beginning researchers in the social sciences. Oxford University Press.
- Kyburz-Graber, Regula. (2004). "Does case-study methodology lack rigour? The need for quality criteria for sound case-study research, as illustrated by a recent case in secondary and higher education", in Environmental Education Research 10(1): 53–65. doi:10.1080/1350462032000173706
- Lijphart, Arend. (1971) "Comparative Politics and the Comparative Method", in The American Political Science Review, 65(3): 682-693. Available from 
- Ragin, Charles C. and Becker, Howard S. eds. (1992) What is a Case? Exploring the Foundations of Social Inquiry. Cambridge: Cambridge University Press. ISBN 0-521-42188-8
- Scholz, Roland W. and Tietje, Olaf. (2002) Embedded Case Study Methods. Integrating Quantitative and Qualitative Knowledge. Thousand Oaks, CA: Sage. ISBN 0-7619-1946-5
- Straits, Bruce C. and Singleton, Royce A. (2004) Approaches to Social Research, 4th ed. Oxford: Oxford University Press. ISBN 0-19-514794-4. Available from: 
- Thomas, Gary. (2011) How to do your Case Study: A Guide for Students and Researchers. Thousand Oaks, CA: Sage.