The Bayley Scales of Infant Development. II: Where to start?

This study examines whether the Bayley Scales of Infant Development-Second Edition (Bayley II) Mental Scale scores vary on the basis of which item set is considered the starting point of an infant's assessment. The Bayley II was administered to 78 12-month-old infants by certified examiners beginning with the 12-month age item set. A second certified examiner then administered 10 additional items that completed the 11-month and 13-month age item sets. Of the 78 infants tested, 73 (94%) met basal and ceiling criteria in all three item sets. Three separate Mental Developmental Index (MDI) scores were calculated using 12-month norms for each subject, which were based on the raw scores generated from the 11-, 12-, and 13-month age item sets. Scores calculated on the 11-month set were significantly lower than those on the 12-month set, which were in turn significantly lower than those on the 13-month set. When tested on the 11-month instead of on the 12-month item set, twice as many infants received lower than normal (< 85) MDI scores, indicating an impact on referability decisions. Minor adjustments in administration of the Bayley II, such as those based on assumptions regarding an infant's current level of functioning, significantly affect infant test scores and eligibility for follow-up services. Standardized use of this test should minimize variability in test scores.