Interaction

Purpose
An interaction exists between Factors when the effect of one Factor upon the Response variable changes depending on the level of the other Factor(s).
Example
Let’s take coffee as our example .
The taste of coffee will change when you add more coffee , but to balance the butter taste of the coffee you may add more water or add more sugar . Thus the effect of the quantity of coffee to the coffee taste changes as you add more sugar or water .

 

Terminology

1. First experimental Factor.
2. Response variable.
3. Response when first Factor is at lowest setting and second Factor is at highest setting.
4. Response when first Factor is at highest setting and second Factor is at highest setting.
5. Response when first Factor is at lowest setting and second Factor is at lowest setting.
6. Response when first Factor is at highest setting and second Factor is at lowest setting.

When no interaction exists between Factors, there is no difference in the Response between the levels of one Factor, when the other factor changes. Significant interaction means that as one Factor changes state from low to high level, the Response changes significantly depending on whether the other Factor is at a high or low setting.
Significant interaction can mask the significance of main effects.