Objectives: The Bayley Scales of Infant and Toddler Development (3rd ed.; Bayley III) are widely used to assess cognitive, language, and motor development of children aged 1-42 months. It is unclear whether or not the reference norms of the Bayley III are acceptable for use in other populations or lead to over- or underestimating the developmental status of target children. This study aimed to compare the Tehran norms to the reference norms.
Materials & methods: We used Bayley III norms to assess cognitive, language, and motor development of 1,674 healthy children from health care centers in Tehran. Differences between the scaled scores were calculated based on the Tehran and reference norms. A one-sample multivariate analysis of variance (MANOVA) was used to control the mean difference scores over all subtests. When MANOVA showed significant differences between the scaled scores based on the Tehran and reference norms, we used univariate analysis to see which subtest and age group led to these significant differences. Finally, the proportions of children with low scores (scaled scores <7 or -1 SD and <4 or -2 SD) based on 2 norms were compared using the McNemar test to determine the over- or underestimation of developmental delay.
Results: The scaled scores based on the Tehran norms varied across values based on the reference norms in all subtests. The mean differences were significant in all 5 subtests (p < .05) with large effect sizes for receptive and expressive communication, fine and gross motor subtests of .20, .23, .14, and .25, respectively, as well as with a small effect size for the cognition subtest of .02. Large effect sizes for all age groups were found for cognition, expressive communication, and fine motor subtests. More children scored below 1 and 2 SD using the Tehran norms. Using the reference norms resulted in underestimation of developmental delay regarding cognitive, receptive and expressive communication, and fine and gross motor skills.
Conclusion: Population-specific norms should be used to identify children with low scores for referral and intervention. The Tehran norms differed from the reference norms for all subtests, and these differences were clinically significant.
Keywords: Bayley Scales of Infant and Toddler Development; Children; Development; Testing norms.