OK, so I find that among Black users, Hotmail is associated with a 1.05 relative risk to the baseline, and among white users, 1.15 relative risk. How do I then apply this knowledge to an incoming user of an unknown race with a Hotmail account? Do I give them the population weighted average of of the relative risks among all my buckets? What if I do know their race? Do Black users get assigned 1.05 as much risk and white users get 1.15?
Also, what if some of my buckets aren't full enough to get a high confidence of the relative Hotmail risk in that bucket? Let's say I don't have enough gay Hispanic women in my cohort. Do I just drop them from the analysis and hope they're similar to the other populations?