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Comparison of the 1969 and 1993 standardizations of the Bayley Mental Scales of Infant Development for infants with Down's syndrome.

The Bayley Scales of Infant Development (BSID) were re-standardized in 1993 (BSID-II). The present study reports a comparison of the two versions with infants with Down's syndrome (DS). The BSID-II was used for 93 assessments of 54 children with DS (age range = 7-43 months). Comparisons were made with the 1969 standardization for 42 of these assessments, and for 45 assessments of 20 typically developing children aged between 6 and 24 months. The 1993 standardization produced significantly lower mean differences of 1.0 months mental age and 8.4 points mental development index for infants with DS, and 1.5 months mental age and 9.2 points mental development index for the typically developing group. Nineteen per cent more infants with DS had scores below two standard deviations and there was a larger decrement for lower functioning children. Both groups of children have to perform at a higher level to achieve the same relative score on BSID-II compared to BSID. This indicates that caution should be used in comparing cohorts of children tested on different versions of the Bayley scales. In addition, concerns are highlighted regarding the rules for establishing basal and ceiling levels for BSID-II for children with developmental delays.