Hello, I have 2 group (sick/healthy), N=30 for each. the other variable is within- two measures time (short/ long). I want to add covariates.

1. Is there is a limit of possible covariates? What is the rule of thumb about the sample size?
2. One of the covariates does not distribute normally- is this is a problem to use it in that case?

I have other analysis: 2 in between group (sick/ healthy) and 4 in between groups (1/2/3/4 measure).

Is there a difference in that sense in the use of the covariates compared to the first analysis?

Thank you,
Tal