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causal inference only if it can establish an uninterrupted causal path linking the putative causes to the observed effects, at the appropriate level(s) of analysis as specified by the theory being tested. Evidence that a single necessary intervening variable along this path was contrary to expectations strongly impugns any hypothesis whose causal effects rely on that causal path alone. The inferential and explanatory value of a causal path is weakened, though not negated, if the evidence on whether a certain step in the putative causal path conformed to expectations is simply unobtainable. Also, theories frequently do not make specific predictions on all of the steps in a causal process, particularly for complex phenomena. When data is unavailable or theories are indeterminate, process-tracing can reach only provisional conclusions.

Another potential problem for process-tracing is that there may be more than one hypothesized causal mechanism consistent with any given set of process-tracing evidence. The researcher then faces the difficult challenge of assessing whether alternative explanations are complementary in the case, or whether one is causal and the other spurious. Even if it is not possible to exclude all but one explanation for a case, it may be possible to exclude at least some explanations and thereby to draw inferences that are useful for theory-building or policymaking.

Olav Njølstad has emphasized this problem in case study research, noting that differing interpretations may arise for several reasons. First, competing explanations or interpretations could be equally consistent with the available process-tracing evidence, making it hard to determine whether both are at play and the outcome is overdetermined, whether the variables in competing explanations have a cumulative effect, or whether one variable is causal and the other spurious. Second, competing explanations may address different aspects of a case, and they may not be commensurate. Third, studies may be competing and commensurate, and they may simply disagree on the facts of the case.

Njølstad offers several useful suggestions on these problems, although we disagree with his suggestion that these are substantially different from the standard methodological advice offered in discussion in Chapter 3. These suggestions include: identifying and addressing factual errors, disagreements, and misunderstandings; identifying all potentially relevant theoretical variables and hypotheses; comparing various case studies of the same events that employ different theoretical perspectives (analogous to careful attention to all the alternative hypotheses in a single case study); identifying additional testable and observable implications of competing interpretations of a single case; and identifying the scope conditions for explanations of a case or category of cases.446

Summary on Process-Tracing

Process-tracing provides a common middle ground for historians interested in historical explanation and political scientists and other social scientists who are sensitive to the complexities of historical events but are more interested in theorizing about categories of cases as well as explaining individual cases. We do not regard process-tracing as a panacea for theory testing or theory development. It can require enormous amounts of information, and it is weakened when data is not accessible on key steps in a hypothesized process. In a particular case, limited data or underspecified theories (or both) may make it impossible to eliminate plausible alternative processes that fit the available evidence equally well. Both false positives, or processes that appear to fit the evidence even though they are not causal in the case at hand, and false negatives, processes that are causal but do not appear to be so, are still possible through measurement error or under-specified or misspecified theories.

Process-tracing has many advantages for theory development and theory testing, however, some of them unique. It is a useful method for generating and analyzing data on