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 longitudinal study allows the researcher to focus on these children as they mature and record developmental patterns across time. A disadvantage of the longitudinal study is that researchers must be engaged in the study over a period of years and risk losing some of their research subjects, who may discontinue their participation for any number of reasons. Another disadvantage of the longitudinal study reflects the fact that some of the changes or behaviors observed during the study may be the effects of the assessment process itself. In addition to the longitudinal study, some researchers may employ the cross-sectional study method. In this method, the subjects, or cohort, are drawn from different groups and are studied at the same point in time.