Longitudinal data analysis (LDA) comprises a set of techniques that can be used to analyse data where the same variable or variables have been measured at different time points. ... There are a number of such methods, each suited to the analysis of different types and combinations of variables. When Longitudinal data looks like a time series is when we measure the same thing over time. The big difference is that in a time series we can measure the overall change in the measurement over time (or by group) while in a longitudinal analysis you actually have the measurement of change at the individual level.