A comparison of the MMPI and the Psychosis Proneness Scales in their classification of normals controls for use in schizophrenia spectrum research.

The original MMPI and five of the Chapmans' Psychosis Proneness Scales (PPS; L. J. Chapman, J. P. Chapman, & Miller, 1982) were administered to college students from a nonclinical setting. Two normal control groups (each with 50 female and 50 male subjects) were formed, one on the basis of item endorsement rates on the PPS and the other on the basis of K-corrected MMPI profiles. Because of the multidimensional nature of the MMPI, as compared to the single-sign nature of the various PPS, we expected most subjects classified as normal on the MMPI to also be classified as normal on the PPS. However, we predicted that a relatively high number of subjects classified as normal on the PPS would produce clinically elevated MMPI profiles. These predictions were supported. The convergent validity strategy revealed that 56% of PPS normal controls produced clinically elevated MMPI profiles, many of which were schizophrenia spectrum related. However, 71% of the MMPI normals had scores in the normal range on all five PPS. None of the MMPI controls produced PPS values associated with spectrum membership.