Multiple-spell, discrete-time survival analysis of developmental data: object concept in pigtailed macaques.

To measure cognitive development in 123 pigtailed macaques, we calculated medians and quartiles for the milestones of classical (Piagetian) object concept development (plain reach, partial-hide, full-hide, and A-not-B) in both 2-dimensional (screen) and 3-dimensional (well) tasks. Using multiple-spell, discrete-time survival analysis, we examined the influence of 4 independent variables (starting age, sex, birth weight, and task) on the probability of completing the milestones on any given test. The significant predictors for completion of the well sequence were birth weight and task. The best predictors for completion of the screen sequence were birth weight, number of test sessions, and interaction between birth weight and task. Birth weight had a greater predictive relationship in the earlier stages. This analysis provided information that cannot be obtained with traditional parametric methods.