Yesterday’s post outlined one issue in deciding whether to put a categorical predictor variable into Fixed Factors or Covariates in SPSS GLM. That issue dealt with how SPSS automatically creates dummy variables out of any variable in Fixed Factors.
Another default to keep in mind is that SPSS will automatically create interactions between any and all variables in Fixed Factors. If you put 5 variables in Fixed Factors, you’ll get all 2-way, 3-way, 4-way, and even a 5-way interaction among those 5 variables.
In contrast, GLM doesn’t create any interactions automatically between Covariates or between Covariates and Fixed Factors.
You can override this default quite easily. Just click on the Model button, then choose “Custom Model.” You can then choose which interactions you do, or don’t, want in the model.
So depending on whether or not you want interactions in the model, and whether your variables are already dummy coded, choosing Covariates or Fixed Factors for your categorical predictors can create a lot less work overriding defaults.
Editor’s Update: If you want to learn in depth how SPSS deals with dummy coding and interactions, how to implement and interpret them, as well as the other options in SPSS GLM, check out our workshop on Running Regressions and ANCOVAs in SPSS GLM. It’s now available in a home study version: http://theanalysisinstitute.com/workshops/SPSS-GLM/index.html