I'm trying to understand the theory behind calculating a two sample t-test with one or more covariates. Specifically three cases are interesting:
- two continuous covariates
- two categorical covariates
- one continuous covariate like age and one categorical covariate like gender
Moreover, the issue of centering covariates in these cases is something I struggle to grasp.
I assume this can be done in the context of the general linear model, like normal t-tests and ANOVA, but I couldn't find much online about the equations or implementation, so I was hoping someone could give me some hints where to read up on this or explain it to me.
Any help appreciated!