A Statistical Consideration Regarding the Threshold of the Psychopathy Checklist–Revised

ABSTRACT

The Psychopathy Checklist–Revised (PCL–R; Hare, 2003) is one of the most commonly used measures of psychopathy. Scores range from 0 to 40, and legal and mental health professionals sometimes rely on a cut score or threshold to classify individuals as psychopaths. This practice, among other things, assumes that all items contribute equally to the overall raw score. Results from an item response theory analysis (Bolt, Hare, Vitale, & Newman, 2004), however, indicate that PCL–R items differ in the amount of information they can provide about psychopathy. We examined the consequences of these item differences for using a cut score, detailing the consequences for a previously applied cut score of 30 as an example. Results indicated that there were more than 8.5 million different response combinations that equaled 30 and more than 14.2 million that equaled 30 or more. This raw score, like others, corresponded to a broad range of PCL–R-defined psychopathy, indicating that applying cut scores on this measure results in imprecise quantifications of psychopathy. We show that by using the item parameters along with an individual's particular scores on the PCL–R items, it is possible to arrive at a more precise understanding of an individual's level of psychopathy on this instrument.

Notes

¹ Lindhiem et al. (2013) applied a method that they dubbed the posterior probability of diagnosis (PPOD). PPOD does the following: Given the IRT item parameters of the symptoms for a particular psychopathological syndrome and given the minimum number of criteria needed to meet the diagnosis, the PPOD determines the posterior probability of a combination of symptoms to yield a theta level on the latent trait that would be concomitant with the minimum theta level afforded by the number of symptoms necessary. Lindhiem et al. chose to use a 2PL framework to accommodate for the dichotomous nature of the DSM diagnostic criteria and a Bayesian perspective to acknowledge the standard errors of the theta parameters.