Longitudinal analysis is the study of a population over time, as opposed to
cross-sectional
analysis which is limited to a single point in time. Examples of
longitudinal analyses conducted by MTPPI include
time-series analysis
and panel studies; in general,
panel surveys use individual-level data and time-series analysis uses
aggregate-level data (see ecological
association). However, the chief difference is that panel studies have a
relatively large number of units studied over a small number of time points
(maybe just two), whilst time-series data have a small number of units
(maybe just one) and a large number of time points. The main advantage of
longitudinal analyses is increased statistical power and the ability to
examine changes within individuals over time.