Yet all you have to do is miss a factor and you've ended up with correlation. As a simple question, what about people who have weak stomachs or hearts? My moth

Yet all you have to do is miss a factor and you've ended up with correlation. As a simple question, what about people who have weak stomachs or hearts? My mother doesn't drink coffee because it "makes her heart beat too hard". How, with no actual medical data for "coffee makes my heart beat hard", do you control for that? Is that something to control for? This is the "caffeine and healthy pregnancy" problem. We know women who consume less than ~200mg of caffeine tend to have healthier pregnancies.. but if you can drink 5+ cups of coffee while pregnant and not get overtaken with nausea, that might indicate something is already wrong.