I want to include two predictors (total brain volume and corrected gray matter volume) into one regression model in order to predict the level of cognition (dependent variable). However, this corrected gray matter volume is calculated as:
gray matter volume / total brain volume = corrected gray matter volume
making the regression model look like this:
Total brain volume + (gray matter volume / total brain volume) = cognition
Is this statistically sounds or are there issues with total brain volume essentially featuring twice in one model?