Hi, I am running RM-ANCOVA in SPSS, where I have an immune marker measured at 2 different time points - this is my within-subject factor.

For my between-subject factor, I have Vit D dosage. I want to look at the interaction between the immune marker at the 2 different time points and Vit D dosage (immune marker*Vit D dosage).

However, when I add age and sex as covariates, the output gives interaction terms for immune marker*age and immune marker*sex.

I have a small sample size, so I don't want this many interaction terms. Is there any way to select which interaction terms you want to run in SPSS (e.g. if I only want interaction with the main effect and not all covariates)?
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