Stability of the Bayley II Scales of Infant Development in a sample of low-risk and high-risk infants.

The aim of this study was to examine the stability of the second edition of the Bayley Scales of Infant Development (Bayley II) by correlating Bayley II scores during the first year of life with Bayley II scores at approximately 18 months of age. Bayley II Mental and Motor Scales were administered to 119 infants (69 males, 50 females) during their first year of life; mean age for low-risk group 7 months (SD 3.01) and mean age for high-risk group 7.8 months (SD 2.46). A second set of Bayley II scores was collected on 104 of the infants at 17 to 22 months of age; mean age for low-risk group 18.5 months (SD 1.07) and mean age for high-risk group 18.9 months (SD 1.11). Mean time interval between the two assessments was 11.5 months for the low-risk group and 11.15 months for the high-risk group. Stability of the scores over time was assessed. Correlations for the Bayley II Mental Developmental Index and Psychomotor Developmental Index between administrations during the first and second years of life were r=0.49 (p<0.001) and r=0.48 (p<0.001) respectively. We conclude that approximately 23 to 24% of the variance in the infants' later Bayley II scores could be explained by the earlier scores. This is one of only two studies to examine the stability of Bayley II over time. Replication studies are needed to evaluate the consistency of these findings across other samples of infants.