We use cookies to ensure that we give you the best experience on our website. You can change your cookie settings at any time. Otherwise, we'll assume you're OK to continue.

Department of Psychology


Publication details for Professor Charles Fernyhough

Jones, S.R. & Fernyhough, C. (2007). Reliability of factorial structure of the Peters et al. delusions inventory (PDI-21). Personality and individual differences 43(4): 647-656.

Author(s) from Durham


The 21-item version of the Peters et al. Delusions Inventory (PDI-21) is a commonly used tool to measure delusional ideation in the normal population. Two recent principal component analyses have concluded that the PDI-21 has a seven-factor structure. Although these studies found identical factors associated with religiosity and grandiosity, the items loading on the remaining five factors, and hence the interpretation of these, differed. Such seven-factor structures of the PDI-21 are beginning to be used in research; however, a consistent item-level seven-factor structure has not been replicated and no data have been reported to support the reliability of such factors. We administered the PDI-21 to a non-psychiatric sample (N = 493). It was found that, with the exception of religiosity/religiousness, the previously reported factors of the PDI-21 had Cronbach’s alphas of less than 0.7. After a factor analysis using principal axis factoring, parallel analysis suggested the extraction of three factors. Of these factors, only one (religiosity/religiousness) was found to be both internally reliable and meaningful. It is concluded that the PDI-21 is best used with a unidimensional scoring system and that new measures are needed to assess specific types of delusion-like beliefs in the normal population.