Longitudinal studies involve repeated observations of the same items over lengthy periods. They are often used in psychology to study developmental trends across the life span, and in sociology to study events throughout lifetimes or generations.

Because they tend to be observational, it has been argued that they may have less power to detect causal relationships than do experiments. But they have the compensating virtue of allowing social scientists to distinguish short from long-term phenomena.

Types include cohort studies and panel studies. Cohort studies sample a cohort, defined as a group experiencing some event (typically birth) in a selected time period; panel studies sample a cross-section, and survey it at (usually regular) intervals.