The MMPI-168(L) and ADD in assessing psychopathology in individuals with mental retardation: between and within instrument associations.
An abbreviated version of the Minnesota Multiphasic Personality Inventory, the MMPI-168(L), modified for use with clients who have moderate or mild mental retardation, was administered to 58 clients, most of whom had co-existing dual psychiatric diagnoses. Another recently developed instrument, the Assessment of Dual Diagnosis (ADD), was administered by interviewing a knowledgeable care giver. Correlations were examined among the raw scores on the 13 ADD scales and T scores of the 13 MMPI-168(L) scales. Contrary to expectations few correlations were found between the scales of the two instruments including scales purported to assess similar psychological constructs. The major exception was the Mania scale of the MMPI-168(L), which correlated moderately well with the Schizophrenia and Dementia scales of the ADD. Client age correlated strongly and negatively with scores on the Conduct Disorder and Sexual Disorder scales of the ADD. Finally, intra-instrument scale correlations were surprisingly large and, from a clinical and diagnostic perspective, meaningful. However, the large number of intra-instrument correlations showed that the scales of both instruments possess considerable overlap, which could make differential diagnosis problematic. It is suggested that it might be necessary to administer both instruments, and carefully consider behavioral history, to accurately diagnose psychiatric disturbances or personality characteristics of individuals with mental retardation.