These relationships simultaneously describe differences between different individuals (why does person 1 save morn than person 2?), and differences in behaviour of a given individual over time (why does person 1 save morn in 1992 than in 1990?). This type of model usually requires panel data, repeated observations over the same units. They are ideally suited for analysing policy changes on an individual level, provided that can be assumed that the structure of the model is constant the (near) future.