Crud (Re)Defined.

The idea that in behavioral research everything correlates with everything else was a niche area of the scientific literature for more than half a century. With the increasing availability of large data sets in psychology, the "crud" factor has, however, become more relevant than ever before. When referenced in empirical work, it is often used by researchers to discount minute-but statistically significant-effects that are deemed too small to be considered meaningful. This review tracks the history of the crud factor and examines how its use in the psychological- and behavioral-science literature has developed to this day. We highlight a common and deep-seated lack of understanding about what the crud factor is and discuss whether it can be proven to exist or estimated and how it should be interpreted. This lack of understanding makes the crud factor a convenient tool for psychologists to use to disregard unwanted results, even though the presence of a crud factor should be a large inconvenience for the discipline. To inspire a concerted effort to take the crud factor more seriously, we clarify the definitions of important concepts, highlight current pitfalls, and pose questions that need to be addressed to ultimately improve understanding of the crud factor. Such work will be necessary to develop the crud factor into a useful concept encouraging improved psychological research.