Method of Difference. Researchers can use the method of difference alone or in conjunction with the method of agreement. The method of difference is usually stronger and is a “double application” of the method of agreement. A researcher first locates cases that are similar in many respects but differ in a few crucial ways. He or she pinpoints features whereby a set of cases are similar with regard to an outcome and causal features. The method of difference reinforces information from positive cases (e.g., cases that have common causal features and outcomes) with negative cases (e.g., cases lacking the outcome and causal features). Thus, a researcher looks for cases that have many of the causal features of positive cases but lack a few key features and have a different outcome.
An Example. The method of agreement and method of difference are difficult to grasp in the abstract. Ragin (1987) provided a system for using the method of agreement and difference. Look at Box 16.2 for an example of each method. In the method of agreement chart, note that a and b are common in all four cases and are crucial similarities despite the many difference (c-q). Suppose cases 1 through 4 represent four nations and the letters represent features of the nations. Thus, a is a democratic form of government, b is a social structure with several equally strong social classes, c is a weak army, d is a large professional army, e is a large drafted army, f is a large “free” public education system, and so on. Using the method of agreement, a researcher interested in explaining outcome a (a democratic government) notes the regularity of b (several equally strong social classes) across all four cases where a democratic form of government developed. He or she hypothesizes that b is a critical causal factor for the development of a.