main effect

When Main Effects are Not Significant, But the Interaction Is

January 21st, 2015 by

If you have significant a significant interaction effect and non-significant main effects, would you interpret the interaction effect?

It’s a question I get pretty often, and it’s a more straightforward answer than most.


Actually, you can interpret some main effects in the presence of an interaction

November 14th, 2014 by

One of those “rules” about statistics you often hear is that you can’t interpret a main effect in the presence of an interaction.

Stats professors seem particularly good at drilling this into students’ brains.

Unfortunately, it’s not true.

At least not always. (more…)

Interpreting Lower Order Coefficients When the Model Contains an Interaction

February 23rd, 2009 by

A Linear Regression Model with an interaction between two predictors (X1 and X2) has the form: 

Y = B0 + B1X1 + B2X2 + B3X1*X2.

It doesn’t really matter if X1 and X2 are categorical or continuous, but let’s assume they are continuous for simplicity.

One important concept is that B1 and B2 are not main effects, the way they would be if (more…)