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Understanding the structure of the Psychopathy Checklist – Revised

An exploration of methodological confusion

Glasgow Caledonian University and Douglas Inch Centre, Glasgow, UK

Christine Michie, BSc

Glasgow Caledonian University, Glasgow, UK

Jennifer Skeem, PhD

University of California, Irvine, California, USA

Correspondence: Dr David J. Cooke, Forensic Psychology Services, Douglas Inch Centre, 2 Woodside Terrace, Glasgow G3 7UY, UK. Email djcooke{at}rgardens.vianw.co.uk

Declaration of interest None.

Funding detailed in Acknowledgements.

¹ 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.

² 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.

³ 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.

⁴ Another problem in the literature is the proliferation of underpowered studies. Confirmatory factor analysis requires moderate-to-large samples (Kline, 1998). Many of the attempts to explore the structure of the PCL measures have been seriously underpowered in terms of sample size, with samples at, or even well below,150 individuals (e.g. Jackson et al,2002; Hill et al,2004; Salekin et al, 2006; Vitacco et al, 2006). Kline (1998) provides guidance on the issue and indicates that 20 cases per free parameter is desirable, 10:1 is just acceptable and the statistical stability with a 5:1 must be regarded as suspect. The three-factor hierarchical model with testlets has 36 free parameters, suggesting a minimum sample size of between 360 and 720. The four-factor hierarchical model has 40 free parameters (minimum n=400-800); the four-factor correlated model has 42 free parameters (minimum n=420-840) and the two-factor, four-facet hierarchical model has 41 free parameters (minimum n=410-820). Underpowered studies will mislead (Floyd & Widaman, 1995). In addition to the problem of lack of stability is the problem of Heywood cases. Small samples are prone to improper solutions in which estimated correlations are greater than 1 or estimated error variances are less than 0. Solutions may also fail to converge.

⁵ The level of fit achieved on the development sample using this method is excellent. S-B ²=167, d.f.=56, AIC=55, NFI=0.98, NNFI=0.98, CFI=0.99, RSMEA=0.04.

⁶ Some commentators have advocated the use of MPlus; the rationale for their preference is unclear. The same pattern of results was achieved using MPlus. The level of fit achieved with the three-factor model with testlets was good: ²=181, d.f.=40, CFI=0.95, RSMEA=0.06; for the three-factor model without testlets it was fair: ²=261, d.f.=0.43, CFI=0.92, RSMEA=0.08; and for the four-factor hierarchical model the fit was poor: ²=692, d.f.=73, CFI=0.82, RSMEA=0.10. Results for all models are available from the authors.

Psychopathy is the key construct in the Dangerous and Severe Personality Disorder (DSPD) Programme. The Psychopathy Checklist - Revised is used as a primary means of selection for the programme. The Checklist confounds two distinct constructs - personality disorder and criminal behaviour. This confound is important both practically and theoretically. For example, under the criteria for DSPD it is necessary to demonstrate that personality disorder has a functional link with future risk of criminal behaviour. The confound has been exacerbated recently by claims that criminal behaviour is a core feature of psychopathic disorder. This contention is based on inappropriate analytical methods. In this paper we examine the source of this confound and illustrate how inappropriate methods can mislead.

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