In repeated measures designs, there are several individuals and measurements are
taken repeatedly on each individual. When these repeated measurements are taken
over time, it is called a longitudinal study or, in some applications, a panel study.
Typically various covariates concerning the individual are recorded and the interest
centers on how the response depends on the covariates over time. Often it is rea-
sonable to believe that the response of each individual has several components: a
fixed effect, which is a function of the covariates; a random effect, which expresses
the variation between individuals; and an error, which is due to measurement or un-