COGNITIVE AND MOTOR SCORE DIFFERENCES IN BIOLOGICALLY AT-RISK INFANTS

Interpretation of the meaning of differences between the Mental Developmental Index (MDI) and Psychomotor Developmental Index (PDI) of the Bayley Scales of Infant Development is unclear. Herein, the relationship between MDI/PDI scores in 517 infants is evaluated at 6, 12, and 24 months in terms of levels of socioeconomic status (SES), biological risk, degree of prematurity, and cognitive ability level (AL). The relationship between MDI and PDI scores also is compared in a longitudinal sample of 224 children. Socioeconomic status did not influence the relationship between MDI and PDI scores, biologic risk affected the relationship between MDI and PDI only at 6 months, and MDI/PDI difference scores varied depending on AL throughout infancy. At higher levels of cognitive function, the MDI routinely was greater than the PDI; at lower cognitive levels, the reverse was true. Correlations between difference scores over time and difference score reliability (relationship between MDI and PDI at each age) generally were weak. Instability and measurement error underscore the need for caution in interpretation of MDI/PDI differences.