The meaning of a measure: p as a general measure of psychopathology

The variable “p” is characterized as a single dimension of risk for psychopathology. The aim of this paper is to critically assess debates over “p” in the scientific literature, and their larger philosophical import. What uses – practical, or theoretical – does such a measure serve? What reasons might one have to be skeptical of p? Drawing upon methodological and empirical critiques of p in the scientific literature, this paper argues that p is not itself explanatory. However, the controversy over p is informative in at least three ways: First, it is illustrative of a larger divide surrounding the value and import of factor analysis in psychiatric research. Second, it provides insight into debates over p and other “open” partial operational definitions or “wide operational” concepts. Third, debates over p shed light on competing research programs in psychiatry that place greater emphasis on dimensional as opposed to categorical approaches to psychopathology (and in particular HiTOP, the hierarchical taxonomy of psychopathology). The debate over p, in other words, is not best understood as a debate over the “reality” of a single common cause of psychopathology; rather, it is a debate over how best to organize research into the mechanisms underlying psychopathology.