The issue of causal comparability in the social sciences underlies matters of both generalization and extrapolation (or external validity). After critiquing two existing interpretations of comparability, due to Hitchcock and Hausman, I propose a distinction between ontological and epistemic comparability. While the former refers to whether two cases are actually comparable, the latter respects that in cases of incomplete information, we need to rely on whatever evidence we have of comparability. I argue, using a political science case study, that in those cases of imperfect information, an epistemic homogeneity criterion can be an adequate justification for generalization.