Hello,

I took a graduate level stats course a few years ago (psychology department), but I am now realizing that there were some things I did not understand. Specifically, I’m trying to clarify the relationship between main effects, interactions, and simple effects in factorial designs.


Interactions and Main Effects are independent – thus any of the following combinations may occur:
1) interaction changes******main effects change
2) interaction changes******main effects same
3) interaction same******main effects change


Simple Effects: combination of the variability of the interaction and one of the main effects – thus the following may occur:
4) simple effects change******interaction changes******main effects change
5) simple effects change******interaction changes******main effects same
6) simple effects change******interaction same******main effects change

I think all of the above is correct? However, if anyone thinks otherwise then I would be interested to know…

I am much less sure about the following combinations and I’m not sure if they could or could not occur:
7) simple effects change******interaction same******main effects same
8) simple effects same******interaction changes******main effects same
9) simple effects same******interaction same******main effects change
10) simple effects same******interaction changes******main effects change

Any thoughts would be greatly appreciated!