I.e. in a cohort study:

Group A (n=1000) - exposure to X - outcome exposed 70/1000 demonstrate Z

Group B (n = 1000) - no exposure -- outcome control 23/1000 demonstrate Z

I know that I can calculate the Risk Ratio to compare Exposure X to control, to see how much more likely the risk is for Group A given exposure X.

However, to complicate matters there is an exposure (Y) that moves across both groups. I.e.:

of Group A 750 also exposed to Y -- 60 demonstrate Z

Group B 300 also exposed to Y -- 16 demonstrate Z

How do I show that Y is an 'independent risk factor' for Z.

I.e. is there a formula I use to show that Y is an independent risk factor for Z?

Thanks!