Towards a conceptual clarification and possible applications of Meehl’s crud factor: A preregistered systematic review

Paul E. Meehl’s crud factor posits that variables in the social and behavioural sciences are never entirely unrelated, encapsulated in the notion that “everything correlates with everything”. This preregistered systematic review examined how the crud factor and Lykken’s related concept of ambient noise have been defined, interpreted, and applied across research fields. We identified 424 works through database searches and citation tracking and analysed conceptual attributes and citation intentions. Among works allowing an inference about the relationship between the two concepts, the crud factor and ambient noise were almost uniformly treated as interchangeable, and both were most frequently invoked to argue that null-hypothesis significance testing against an exact-zero effect is unsuitable for corroborating theories. Reported numerical conventions for the crud factor varied widely, indicating no established empirical benchmark. Based on the synthesis, we propose an operational definition within the multitrait-multimethod validation framework: narrow-sense crud comprises heterotrait-heteromethod associations unexplained by substantive theory or theories of methodological artifacts, whereas broad-sense crud additionally encompasses heterotrait-monomethod associations and thus method-related covariance. This distinction provides a basis for empirically estimating the magnitude of the crud factor and evaluating its methodological implications.