Research method used to study changes over time.

Researchers in such fields as developmental psychology use longitudinal studies to study changes in individual or group behavior over an extended period of time by repeatedly monitoring the same subjects. In longitudinal research, results are recorded for the same group of subjects, referred to as a cohort, throughout the course of the study.

An example of a longitudinal study might be an examination of the effects of preschool attendance on later school performance. The researchers would select two groups of children—one comprised of children who attend preschool, and the other comprised of children who had no preschool experience prior to attending kindergarten. These children would be evaluated at different points during their school career. The...