Case Studies and Theory Development in the Social Sciences - Alexander L. George [45]
Second, the research objective chosen in a study may require comparison of several cases. There are several comparative research designs. The best known is the method of “controlled comparison”—i.e., the comparison of “most similar” cases which, ideally, are cases that are comparable in all respects except for the independent variable, whose variance may account for the cases having different outcomes on the dependent variable. In other words, such cases occupy neighboring cells in a typology, but only if the typological space is laid out one change in the independent variable at a time. (See Chapter 11 on typological theories.)
As we discuss in Chapter 8 on the comparative method, controlled comparison can be achieved by dividing a single longitudinal case into two—the “before” case and an “after” case that follows a discontinuous change in an important variable. This may provide a control for many factors and is often the most readily available or strongest version of a most-similar case design. This design aims to isolate the difference in the observed outcomes as due to the influence of variance in the single independent variable. Such an inference is weak, however, if the posited causal mechanisms are probabilistic, if significant variables are left out of the comparison, or if other important variables change in value from the “before” to the “after” cases.
However, even when two cases or before-after cases are not perfectly matched, process-tracing can strengthen the comparison by helping to assess whether differences other than those in the main variable of interest might account for the differences in outcomes. Such process-tracing can focus on the standard list of potentially “confounding” variables identified by Donald Campbell and Julian Stanley, including the effects of history, maturation, testing, instrumentation, regression, selection, and mortality. 173 It can also address any idiosyncratic differences between the two
Another comparative design involves “least similar” cases and parallels John Stuart Mill’s method of agreement.174 Here, two cases are similar in outcome but differ in all but one independent variable, and the inference might be made that this variable contributes to the invariant outcome. For example, if teenagers are “difficult” in both postindustrial societies and tribal societies, we might infer that their developmental stage, and not their societies or their parents’ child-rearing techniques, account for their difficult natures. Here again, left-out variables can weaken such an inference, as Mill recognized, but process-tracing provides an additional source of evidence for affirming or infirming such inferences.
Another type of comparative study may focus on cases in the same cell of a typology. If these have the same outcome, process-tracing may still reveal different causal paths to that outcome. Conversely, multiple studies of cases with the same level of a manipulable independent variable can establish under what conditions that level of the variable is associated with different outcomes. In either approach, if outcomes differ within the same type or cell, it is necessary to look for left-out variables and perhaps create a new subtype.
Often, it is useful for a community of researchers to study or try to identify cases in all quadrants of a typology. For example, Sherlock Holmes once inferred that a dog that did not bark must have known the person who entered the dog’s house and committed a murder, an inference based on a comparison to dogs that do bark in such circumstances. To fully test such an assertion, we might also want to consider the behavior of non-barking non-dogs on the premises (was there a frightened cat?) and barking non-dogs (such as a parrot). The process of looking at all the types in a typology