Various CHA scholars have contributed to causal process tracing and helped establish it as principal alternative to VBA. A recent, Bayesian-informed version is particularly relevant to CHA because it shares the same historiographical sensibilities of seeing knowledge evolve through a close dialogue between new findings and the existing foreknowledge. It makes causal inferences conditional on a pre-testing articulation of testworthy hypotheses and juxtaposes new test results onto older ones at the post-testing stage. Process tracing defines the testworthiness of hypotheses according to the number and diversity of empirical implications a theory has before the testing starts. It also defines test strength in terms of the specificity of the hypotheses that are tested and of their uniqueness vis-à-vis the alternative explanations against whichthey are paired. The chapter introduces a new tool, the theory ledger, to help evaluate test strength and to update confidence in causal inferencing.

This chapter generalizes Van Evera’s typology of process tracing tests to a fully probabilistic context, introducing measures of anticipated test strength that will also be used in Chapter 11.