Including the interaction but not the main effects in a model
I have an experimental design with pretest and posttest. Two groups, experimental and control group. I am interested in finding whether there will be a difference between experimental and control group at the posttest, controlling for pretest. Therefore, I run ANCOVA with pretest as covariate, posttest as DV, and group as IV.
I am also interested to see if there will be a gender differences for the experimental effect. Thus, I am interested in looking at the interaction term group*gender. I am not interested in looking at gender main effect alone, as it doesn't mean much to this experimental design.
I customized the model and excluded the main effect of gender, and the results showed only the main effect of group and the interaction effect of group*gender.
Is this okay, or do I need to run a full model including gender main effect? Thank you.