I'm running a 2x2 factorial ANCOVA, where the factors are either being in condition A or B and either being part of a certain group or not, with two covariates being age and gender. When I run the full factorial model, one of the main effects is significant but the interaction is not.
However, when I build the model and only include the interaction and covariate, and do not include main effects, the interaction is significant.
If I am only interested in the interaction, can I interpret it as significant, or must I include the main effects too?
Thanks for your help!