Final answer:
Comparing the same group of people on the same measures over a lengthy period defines longitudinal research, which involves repeated observations of the same variables.
Step-by-step explanation:
Comparing the same group of people on the same measure(s) over a lengthy time period defines longitudinal research. Longitudinal research involves following a group of research participants for an extended period of time, administering the same tests or observations repeatedly. For instance, if we wanted to study the dietary habits of individuals as they age, we could survey them about their eating patterns at age 20, then again at 30, and once more at 40, to see how their habits have evolved over time. This is in contrast to cross-sectional research, which would compare different age groups at a single point in time, risking confounding generational differences with age-related changes.