R Statistics Cookbook

Repeated measures

Repeated measures designs contain several measurements for the same experimental unit. This generally occurs when we assign different doses of a treatment to a unit and want to evaluate the response through time.

We now have a fixed effect for time, which is typically coded as 1,2,3..,k that needs to be added. We would expect that this variable/effect will have an impact on the response.

These designs raise four major points, and we need to decide how to solve them:

• Do we think that time has the same impact for all of the experimental units? We have two options for this:
  • If yes, we will just add the time variable as a fixed effect.
  • If no, we will still add the time variable as a fixed effect, plus a random effect for time ...