Sampling (case studies)

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A common misunderstanding about case study research is that one cannot generalize from a case study. However, through information-oriented sampling one may arrive at case studies that allow generalization. When the objective is to achieve the greatest possible amount of information on a given problem or phenomenon, a random sample or representative case may not be the most appropriate strategy. This is because the typical or average case is often not the richest in information. Extreme or atypical cases often reveal more information.

The following three types of information-oriented sampling may be distinguished: (1) Extreme or deviant case sampling, (2) Critical case sampling, and (3) Paradigmatic case sampling. The extreme case can be well-suited for getting a point across in an especially dramatic way, which often occurs for well-known case studies such as Freud’s ‘Wolf-Man’ and Foucault’s ‘Panopticon.’

A critical case can be defined as having strategic importance in relation to a general problem. For example, an occupational medicine clinic wanted to investigate whether people working with organic solvents suffered brain damage. Instead of choosing a representative sample among all those enterprises in the clinic’s area that used organic solvents, the clinic strategically located a single workplace where all safety regulations on cleanliness, air quality, and the like, had been fulfilled. This model enterprise became a critical case: if brain damage related to organic solvents could be found at this particular facility, then it was likely that the same problem would exist at other enterprises which were less careful with safety regulations for organic solvents. Via this type of strategic sampling, one can save both time and money in researching a given problem. Another example of critical case sampling is the strategic selection of lead and feather for the test of whether different objects fall with equal velocity. The selection of materials provided the possibility to formulate a generalization characteristic of critical cases, a generalization of the sort, ‘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.’

A paradigmatic case may be defined as an exemplar or prototype. For instance, Thomas Kuhn has shown that the basic skills, or background practices, of natural scientists are organized in terms of ‘exemplars’ or 'paradigms' the role of which in the scientific process can be analyzed. Kuhn showed that such scientific paradigms cannot be expressed as rules or theories. There exists no predictive theory for how predictive theory comes about. A scientific activity is acknowledged or rejected as good science by how close it is to one or more exemplars; that is, practical prototypes of good scientific work. A paradigmatic case of how scientists do science is such a prototype. It operates as a reference point and may function as a focus for the founding of schools of thought.

Galileo’s rejection of Aristotle’s law of gravity was based on 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 fall. And that was what Galileo did. Historians of science continue to discuss whether Galileo actually carried out the famous experiment from the leaning tower of Pisa, or whether it is simply a myth. In any event, Galileo’s experimentalism did not involve a large random sample of trials of objects falling from a wide range of randomly selected heights under varying wind conditions, and so on. Rather, it was a matter of a single experiment, that is, a case study, if any experiment was conducted at all.

Galileo’s view continued to be subjected to doubt, however, and the Aristotelian view was not finally rejected until half a century later, with the invention of the air pump. The air pump made it possible to conduct the ultimate experiment, known by every pupil, whereby a coin or a piece of lead inside a vacuum tube falls with the same speed as a feather. After this experiment, Aristotle’s view could be maintained no longer. What is especially worth noting, however, is that the matter was settled by an individual case due to the clever choice of the extremes of metal and feather. One might call it a critical case: for if Galileo’s thesis held for these materials, it could be expected to be valid for all or a large range of materials. Random and large samples were at no time part of the picture. Most skilled scientists simply do not sample this way with this type of problem.

The case study is 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.'

[edit] See Also

• Sampling (statistics)

[edit] Sources and further reading

• B: Flyvbjerg, "Five Misunderstandings About Case Study Research." Qualitative Inquiry, vol. 12, no. 2, April 2006, 219-245.
• Charles C. Ragin and Howard S. Becker, eds., What is a Case? Exploring the Foundations of Social Inquiry (Cambridge: Cambridge University Press, 1992).