Longitudinal Research

Longitudinal research is generally defined as studies that investigate change over time with the same cohort(s). This definition characterizes four key features of longitudinal research.

First, by change, it means that longitudinal studies focus on a dynamic process rather than a static status as the outcome variable. A process is a series of changes from the initial status to the final status through various intermediate statuses. In a methodological term, one status is often called one wave. Thus, longitudinal studies use multiple waves to represent a process.

Second, by over time, it means that longitudinal studies must include the time dimension as the fundamental predictor. A multiwave process can be considered as a function of time, and various substantive predictors (e.g., gender or IQ scores) can be used to explain the process, as in nonlongitudinal studies.

Third, by with the same cohort(s), it means that longitudinal studies typically measure the same group of individuals (i.e., one cohort) repeatedly over time to examine the intraindividual changes (e.g., each student's reading development) as well as interindividual differences (e.g., gender differences in reading development). In some cases, a longitudinal study repeatedly measures just one same individual (e.g., the single-subject study) or multiple cohorts (e.g., the [Page 617]cohort-sequential study). Longitudinal studies focus on changes within the same cohorts, whereas cross-sectional studies focus on differences across different cohorts.

Fourth, by investigate, it means that longitudinal studies are not limited to repeated measurement or analysis of change over time. Instead, longitudinal design, longitudinal measurement, and longitudinal analysis represent not only the three areas of longitudinal research methodology as an established field of study but also three major phases of a longitudinal study.

Learning, teaching, education, development, intervention, and various psychological processes are essentially intended to produce desirable changes among human beings. Thus, focusing on change over time, longitudinal research can be considered the method of choice for the social and behavioral sciences in general and educational psychology in particular. This entry presents basic methodological issues of longitudinal design, measurement, and analysis and provides real-life examples of empirical longitudinal studies.