What is the main feature of a longitudinal design?

A longitudinal design is characterized primarily by the collection of data from multiple time points. This approach allows researchers to observe changes and developments in a population or phenomenon over time. By measuring the same variables at different intervals, longitudinal studies can demonstrate trends, causality, or the long-term effects of interventions, making them particularly valuable in understanding patterns of behavior, psychological changes, or the progression of psychological conditions.

In contrast, assessing a single point in time is indicative of cross-sectional designs, which provide a snapshot view rather than a temporal analysis. Using a heterogeneous population can be part of your study design depending on other factors but does not specifically define longitudinal designs. Similarly, conducting a study over a short duration is not a feature of longitudinal studies; in fact, longitudinal designs generally require an extended timeframe to effectively track changes.