Criterion-related diagnostic validity and test-retest reliability of the MMPI-168(L) in mentally retarded adolescents and adults.

Sixty-three persons residing in an institution and diagnosed with mild or moderate mental retardation were assessed with a modification of the MMPI-168. Fifty-one of the residents also had psychiatric disturbances. The residents were divided into three categories on the bases of pre-existing DSM III-R diagnoses. The categories were "psychotic," "executive control dysfunction," and "other-control." Significant differences were observed between the psychotic group and the other-control group on scales F, K, 6, and 8. Reevaluations were conducted four to 22 months later (MDN = 12 months). Significant test-retest reliability was found with scales L, F, K, 1, 4, 5, 6, 7, 8, and 9. The MMPI-168(L) can serve as an objective instrument in the assessment of thought disturbance and personality in mentally retarded persons.