I have two similar issues. I have two separate questions, one has a binary DV (is there a brain response yes/no) and the other a continuous DV (how widespread the response is). Each question is looking at the effect of a few IVs on each of the DVs. The IVs are age (continuous), head circumference (continuous) and sex (binary). I am working with infant data and as expected the age and HC are highly correlated.
What I want to do is look at the the effect of age and sex on the two DVs (separately) while controlling for HC. First, is the needed since I know HC and age are correlated (r=.74) so wouldn't controlling for HC of course remove any relationship that may have been there with age and the DV? Would it be best to look at the effect of HC and if there is none, leave it out as a covariate and instead interpret any age effects with HC in mind?
Also, which test is appropriate for this. Before controlling for HC I was using a logistic regression for the binary DV which allows for both continuous and binary IVs. For the continuous DV, I was using a linear regression. Is this even correct? I don't know which test allows me to input a continuous DV and both continuous and binary IVs, let alone a test that allows for that while also inputting a continuous covariate. I use SPSS but if anyone has a specific test in mind I should be able to find a way to do it if SPSS can't.
Thanks in advance!