• Choose a category
• All
• Classic-Fiction

This logic of causal inference for small-n comparisons is highly problematic if the phenomenon being investigated has complex, multiple determinants rather than—as in the simple examples of Mill’s methods discussed above—a single independent variable of presumed causal significance. Thus, in the example of the method of agreement cited above, the investigator might eventually discover that a condition that was “eliminated” as being neither necessary nor sufficient was in fact associated with the outcome when and only when an additional condition, one not included in the initial study, was also present. Meanwhile, failure to discover this additional condition might lead the investigator to prematurely discard the first condition’s significance on the ground that it was not always associated with the type of outcome in question. This highlights the possibility of “false negatives” when applying the logic of elimination that goes along with the other possibility, already alluded to, of false positives.305

It has been argued that, nonetheless, Mill’s methods are useful tools for eliminating causes that are neither necessary nor sufficient conditions. Scholars do seem to use Mill’s methods for this purpose. However, it is not always clear whether they recognize that the variables excluded as neither necessary nor sufficient may still have considerable causal significance when combined with other variables. This is a matter of considerable importance since we believe that there are few nontrivial theories in the social sphere strong enough to support general claims of necessity or sufficiency for single variables, and that indeed the causation of many phenomena of interest to social science researchers is complex and lacks nontrivial necessary or sufficient conditions.

Another major difficulty in employing the logic of elimination occurs when different instances of the phenomenon under investigation have alternative determinants—what Mill referred to as the problem of “plurality of causes.” This condition is termed “equifinality” in general systems theory and is also sometimes called “multiple causality.” Equifinality is present in many social phenomena. For such phenomena, the same type of outcome can emerge in different cases via a different set of independent variables. With the method of agreement we cannot be certain that the outcome is associated only with a given independent variable. If that phenomenon is subject to plurality of causes, we may sooner or later encounter one or more additional cases in which the outcome occurs in the absence of the conditions with which it was earlier associated.306

Some specialists on comparative method have proposed another variant of Mill’s methods, which they call the “indirect method of difference.” Charles Ragin describes this variant as involving “a double application of the method of agreement.” First, the investigator identifies instances of a similar outcome of a phenomenon to see if they display a similar independent variable. If they do, then cases in which that outcome is absent are examined to see if they lacked the independent variable associated with the outcome. Ragin discusses the uses and limitations of this indirect method, noting that it “suffers some of the same liabilities as the method of agreement in situations of multiple causation” as well as with phenomena that are affected by “conjunctural causation.” 307 More generally, Ragin issues a useful warning against “mechanical” application of Mill’s methods.

Some research designs use both the methods of agreement and difference, such as States and Social Revolutions by Theda Skocpol, and Shaping the Political Arena by Ruth Berins Collier and David Collier. The Colliers describe the methodology of their important study as having two components: they combine Mill’s methods of agreement and difference with process-tracing over time within each country to further probe explanations. 308

There has been considerable controversy