Facts to Remember about Interactions
1. An interaction between two factors can be measured only if the two factors are
tested together in the same experiment.
2. When an interaction is absent, the simple effect of a factor is the same for all
levels of the other factors and equals the main effect.
3. When interactions are present, the simple effect of a factor changes as the level of
the other factor changes. Therefore, the main effect is different from the simple
effect