I am examining how sex affects the relationship between 2 variables. So, my model looks like: $$ y \sim sex * x $$ However, there is a known effect of menstrual cycle on $y$ (obviously in females only). Is there a way that I could include this as a covariate in the analysis, while still keeping both groups?
Let's say in females I have factor $m$ that is either 0 (follicular phase) or 1 (luteal phase), representing where they are in their cycle. Would my model be as simple as: $$ y \sim m + sex * x $$ where I would code all men as 0 for the covariate $m$?
Would it matter if the covariate was continuous?