The PCL-R is a 20-item rating scale of traits and behaviours intended for use in a range of forensic settings. Definitions of each item are provided and evaluators rate the lifetime presence of each item on a 3-point scale (0, absent; 1, possibly or partially present; 2, definitely present) on the basis of an interview with the participant and a review of case history information. Although we give a broad verbal account of each of the chief models that have been proposed for the PCL-R (two-, three- and four-factor; see Fig. 2), given the imprecision of natural language, we stress the importance of consulting the mathematical code provided in the data supplement to the online version of this article to specify each model. It is noteworthy that this is only one possible interpretation of this set of covariances: this model has six equivalent models, all of which are equally tenable statistically but which lead to different theoretical interpretations.