Examination of the Item Structure of the Alberta Infant Motor Scale

Abstract

Purpose The Alberta Infant Motor Scale (AIMS) is a screening tool for identifying delayed motor development from birth to 18 months of age. The purpose of this study was to examine the psychometric structure of the AIMS, including the hierarchical scale of items and the precision for measuring infant ability at different ages. Methods Ninety-seven infants with varying degrees of risk of developmental disability were recruited from three hospitals or from the community in the Chicago metropolitan area. Infants were tested on the AIMS at three, six, nine, and 12 months of age. The hierarchical structure and the range and distribution of item difficulty on the AIMS were analyzed using Rasch psychometric analysis. Results The Rasch analysis confirmed that items for each of the four testing positions (supine, prone, sitting, and standing) were arranged in increasing order of difficulty, but a ceiling effect was present. Gaps exist at six ability levels, indicating low precision of measurement for differentiating among infants after about nine months of age. Conclusions The AIMS shows a ceiling effect, measures infant ability best from three to nine months of age, and has few items available for discriminating among infants after they pass the controlled lowering through standing item. Clinical impressions should be drawn with caution at ages when the precision of measurement is low.