Measuring Delusional Ideation: The 21-Item Peters et al. Delusions Inventory (PDI)

There is increasing evidence that in the general population there are schizotypal traits and symptoms that can be measured psychometrically. Norms are reported for a new 21-item version of the Peters et al. Delusions Inventory (PDI; Peters et al. 1999b). The PDI, originally based on the Present State Examination, incorporates the multidimensionality of delusions by including measures of distress, preoccupation, and conviction. A total of 444 healthy individuals completed the 21-item PDI and two other questionnaires measuring florid delusions and social desirability. A subsample also filled out an in-depth schizotypal personality scale. Thirty-three deluded inpatients also completed the PDI. The PDI's psychometric properties confirmed that it remains a reliable and valid instrument to measure delusional ideation in the general population. Consistent with the 40-item PDI, it was normally distributed, no sex differences were found, and there was an inverse relationship with age. Individual items were endorsed by just over one in four healthy adults. Although the deluded sample scored significantly higher, the range of scores overlapped considerably, with 11 percent of healthy adults scoring higher than the mean of the deluded group. As with our previous findings, the two samples were differentiated by their ratings on the distress, preoccupation, and conviction scales. These results suggest that these dimensions may be more important than the content of belief alone for placing an individual on the continuum between normal and delusional thinking.