In a nonrandomized or observational study, propensity scores may be used to balance observed covariates and trajectory groups may be used to control baseline or pretreatment measures of outcome. The trajectory groups also aid in characterizing classes of subjects for whom no good matches are available and to define substantively interesting groups…

An aim of many students of human development is to penetrate behavioral variability in such a way as to reveal stable phenomena. This broad aim is challenged on numerous fronts when researchers study infant behavior, for students of infants are everywhere confronted with variability. But, in the context of research, variability of subjects'…