# ANOVA with 4 factors, 1 covariate & 500 subjects

I'm using SPSS and this drop-down menu: Analyze -> General Linear Model -> Univariate.

DV:                 Number of times patient visited doctor over one year
Factor 1: 2 levels: Two different drugs
Factor 2: 2 levels: Gender
Factor 3: 4 levels: Race
Factor 4: 3 levels: drug taken morning, afternoon, evening
Covariate:          Age (continuous)


There are 500 subjects. Only 10 missing values, all in the factor with 4 levels.

The main effect I'm really interested in is for Factor 1. In a 1-way ANOVA it was highly significant. When I added the next 3 factors and the covariate in a full-factorial model it became not significant (none of the factors were, though when I had just 3 factors + the covariate 2 of the factors were significant).

• Have I included too many factors (+ the covariate) to expect a significant main effect for any of the factors? Somehow "overloaded" the model?
• When I tried a "custom" model, removing all the interactions from the model, 2 of the factors were now significant, which is what I expected. Is that legitimate?
• Your total 500 doesn't matter nearly as much as the size of each cell. Nevertheless, it's probably not causing the problem you have (see my answer) but it is more important than the total N to report. – John May 15 '13 at 11:18