The statistical analyses for achieving these objectives used recently developed statistical methods based on the general linear mixed model. By design, these methods for longitudinal data handled unbalanced data with missing values, incorporated time-varying covariates, and provided more exactly the extent to which the level of lung function of individual children was maintained relative to that of other study children and determined the value of preschool spirometric measurements as predictors of levels of spirometric performance during later childhood and adolescence. These analyses utilized methods developed by Stewart, Pekow, Burchinal, and Helms in 1991 that were unique in that they were designed to handle incomplete, inconsistently-timed data as well as complete, balanced data.