 An early model of explanatory reduction due to William Wimsatt (1976b) assumes that reduction is the causal explanation of an individual event (rather than a generality), involving a “compositional redescription” of a higher level state of affairs into some of its component features. This model is related to Wesley Salmon’s (1971) account of statistical explanation, which uses the notion of  screening off  to explicate the idea of one property being statistically more relevant (and thus more explanatory) than another one. Wimsatt replaces Salmon’s statistical relevance with the idea of causal relevance (as explanatory reduction in biology involves the search for causally relevant factors), and he develops the notion of “effective screening off”. Compositional redescription and explanation in terms of lower level properties can have a greater “cost” of explanation (e.g., by being more complicated) than the description in terms of the higher level property that is effectively screened off. A consequence of the epistemic “cost” requirement is that an explanatory reduction that effectively screens off a described higher level feature does not make it irrelevant and thus eliminable (as in Salmon’s account) because it can be retained for pragmatic purposes.