Reliability of factorial structure of the Peters et al. delusions inventory (PDI-21)

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.