According to Menard (1991), Longitudinal Research is research in which
data are collected for two or more distinct periods (implying the notion of repeated measurements);
the subjects or cases analyzed are the same, or at least comparable, from one period to the next; and
the analysis involves some comparison of data between or among periods.
This type of research is often conducted to explore how phenomena change over time.
For more information on longitudinal research, review the resources below: