What is the purpose of Bayley-III standard scores and what is their typical mean and standard deviation?

• A. To measure growth in height and weight; mean is 50 with SD of 15.
• B. To compare a child’s performance to age peers; mean is 100 with SD of 15.
• C. To categorize risk into normal or abnormal; mean 0 with SD of 1.
• D. To assess behavior in social contexts; mean 50 with SD of 20.

Answer: (B)

Bayley-III standard scores are designed to place a child’s performance in the context of age-based norms, so you can see how they compare to typical peers of the same age. These scores are scaled so the average for any given age is 100 with a standard deviation of 15. That setup makes interpretation straightforward: a score around 100 means typical development for that age, and each 15-point change represents one standard deviation away from the average. This framework helps identify areas where a child is developing on schedule versus areas that may warrant further evaluation, across cognitive, language, and motor domains.

They aren’t about measuring physical growth like height and weight—that’s what growth charts are for. They aren’t expressed as a z-score with a mean of 0 and SD of 1, though you could convert if needed. They also aren’t primarily a tool for assessing behavior in social contexts, though Bayley-III does include broader developmental scales. So the purpose is to compare a child’s performance to age peers, with a mean of 100 and a standard deviation of 15.