A p-value for Process Tracing and other N=1 Studies

We introduce a method for calculating \(p\)-values to test causal hypotheses in qualitative research \emph{a la} process tracing. As in an experiment, our \(p\)-value tells us how often one would make the same or more compelling observations favoring one theory while entertaining a rival theory. We adapt Fisher's (1935) randomization-based urn model to the reality of qualitative researchers, who cannot randomize history, but can make observations about historical processes. Our test includes a method of sensitivity analysis which allows researchers to account for the possibility of observation bias, as well as a framework for representing the varying strenght of individual pieces of evidence, altoguether informing the robustness of qualitative causal inefernce. We provide simulations and replications of previously published work to illustrate how to execute our test using any type of qualitative data about events that took place within one case. This approach adds to the pluralistic turn in the use of probability theory in theory-testing process tracing by offering a simple model with provable conservatism, while relying on few assumptions the consequences of which can be directly assessed.