Windows of achievement for development milestones of Sri Lankan infants and toddlers: estimation through statistical modelling

BACKGROUND: The normative age ranges during which cohorts of children achieve milestones are called windows of achievement. The patterns of these windows of achievement are known to be both genetically and environmentally dependent. This study aimed to determine the windows of achievement for motor, social emotional, language and cognitive development milestones for infants and toddlers in Sri Lanka. METHODS: A set of 293 milestones identified through a literature review were subjected to content validation using parent and expert reviews, which resulted in the selection of a revised set of 277 milestones. Thereafter, a sample of 1036 children from 2 months to 30 months was examined to see whether or not they had attained the selected milestones. Percentile ages of attaining milestone were determined using a rearranged closed form equation related to the logistic regression. The parameters required for calculations were derived through the logistic regression of milestone achievement statuses against ages of children. These percentile ages were used to define the respective windows of achievement. RESULTS: A set of 178 robust indicators that represent motor, socio emotional, language and cognitive development skills and their windows of achievement relevant to 2 to 24 months of age were determined. Windows of achievement for six gross motor milestones determined in the study were shown to closely overlap a similar set of windows of achievement published by the World Health Organization indicating the validity of some findings. CONCLUSIONS: A methodology combining the content validation based on qualitative techniques and age validation based on regression modelling found to be effective for determining age percentiles for realizing milestones and determining respective windows of achievement.