COGNITIVE AND MOTOR SCORE DIFFERENCES IN BIOLOGICALLY AT-RISK INFANTS

Interpretation of the meaning of differences between the Mental Develo pmental Index (MDI) and Psychomotor Developmental Index (PDI) of the B ayley Scales of Infant Development is unclear. Herein, the relationshi p between MDI/PDI scores in 517 infants is evaluated at 6, 12, and 24 months in terms of levels of socioeconomic status (SES), biological ri sk, degree of prematurity, and cognitive ability level (AL). The relat ionship between MDI and PDI scores also is compared in a longitudinal sample of 224 children. Socioeconomic status did not influence the rel ationship between MDI and PDI scores, biologic risk affected the relat ionship between MDI and PDI only at 6 months, and MDI/PDI difference s cores varied depending on AL throughout infancy. At higher levels of c ognitive function, the MDI routinely was greater than the PDI; at lowe r cognitive levels, the reverse was true. Correlations between differe nce scores over time and difference score reliability (relationship be tween MDI and PDI at each age) generally were weak. Instability and me asurement error underscore the need for caution in interpretation of M DI/PDI differences.