“For example, we have been told that those of us who drink a moderate amount of alcohol tend to be in better health. That is a correlation. Does this mean drinking a moderate amount will improve one’s health—a causation? Perhaps not. It could be that good health causes people to drink a moderate amount. Social scientists call this reverse causation. Or it could be that there is an independent factor that causes both moderate drinking and good health. Perhaps spending a lot of time with friends leads to both moderate alcohol consumption and good health. Social scientists call this omitted-variable bias.”