Consider the following hypothetical experiment: A total of four rats are run in a maze three trials a day for two days. The number of wrong turns on each trial are shown below.

The sources of variation in this design are: Subjects, Days, Trials, the Days x Trials interaction, and Error. The ANOVA summary table is shown below. Notice that there are three error terms: one for the effect of Days (MSE = 1.38), one for the effect of Trials (MSE = 0.61), and one for the interaction (MSE = 0.17).