Shane Littrell writes:
I’ve recently graduated with my Masters in Science in Research Psych but I’m currently trying to get better at my stats knowledge (in psychology, we tend to learn a dumbed down, “Stats for Dummies” version of things). I’ve been reading about “suppressor effects” in regression recently and it got me curious about some curious results from my thesis data.
I ran a multiple regression analysis on several predictors of academic procrastination and I noticed that two of my predictors showed some odd behavior (to me). One of them (“entitlement”) was very nonsignificant (β = -.05, p = .339) until I added “boredom” as a predictor, and it changed to (β = – .10, p = .04).
The boredom predictor also had an effect on another variable, but in the opposite way. Before boredom was added, Mastery Approach Orientation (MAP) was significant (β = -.17, p = .003) but after boredom was added it changed to (β = -.05, p = .335).
It’s almost as if Entitlement and MAP switched Beta values and significance levels once Boredom was added.
What is the explanation for this? Is this a type of suppressor effect or something else I haven’t learned about yet?
My reply: Yes, this sort of thing can happen. It is discussed in some textbooks on regression but we don’t really go into it in our book. Except we do have examples where we run a regression and then throw in another predictor and the original coefficients change. When you add a predictor the model changes so it makes sense that the coefficients change too.