A Bayes factor can be used in process tracing to reason about the probative value of evidence for a working hypothesis against a rival (Fairfield and Charman 2022). However, relying on the researcher's own sense of probative strength may inflate confidence in $H_1$ (Zaks 2021). We show how to fully specify a generative model of observations to compute Bayes factors in process tracing: First, researchers count the evidence. Second, they classify it as favoring either hypothesis. Third, they formalize a data-generating process that makes the evidence more expected given $H_1$ and another for a rival $H_R$. The ratio of these probabilities yields a fully specified Bayes factor. We compute two Bayes factors under different assumptions, yielding a Bayes factor range that allows researchers to decide whether the balance of evidence favors $H_1$ considering probability thresholds. Our method also facilitates sensitivity analyses of common assumptions, including those about bias and the probative weight of a smoking gun.

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