Fixed Effects Regression Methods for Longitudinal Data Using SAS

1.3. Costs and Benefits of Fixed Effects Methods

As already noted, the major attraction of fixed effects methods in nonexperimental research is the ability to control for all stable characteristics of the individuals in the study, thereby eliminating potentially large sources of bias. Within-subject comparisons have also been popular in certain kinds of designed experiments known as changeover or crossover designs (Senn 1993). In these designs, subjects receive different treatments at different times, and a response variable is measured for each treatment. Ideally, the order in which the treatments are received is randomized. The objective of the crossover design is not primarily to reduce bias, but to reduce sampling variability and hence produce ...