[jefftk]

Unfortunately our level of certainty generally falls off as we increase the granularity. For example, imagine the patient is a 77yo Polish-American man, and we're lucky enough to have one historical result for 77yo Polish-American men. That man got treatment A and did better than expected. But say if we go out to 70-79y white men we have 1,000 people, of which 500 got treatment A and generally did significantly worse than the 500 who got treatment B. While the more granular category gives us a little information, the sample size is so small that we would be foolish to discard the less granular information.

[mb7733]

This is all true. I originally added a disclaimer to my post that said "assuming you have enough data to support the level of granularity" but I removed it for brevity because I thought it was implied -- small sample size isn't part of Simpson's paradox. My apologies for being unclear