An article published in September 2006 was called "No Booze? You May Lose: Why Drinkers Earn More Money Than Nondrinkers. Researchers reported that drinkers earn 10-14% more than people who do not drink. In an attempt to explain why this might be the case, the study suggests that drinkers are more social than non-drinkers, and that greater social networks lead to higher earnings. What kind of a study was this? Is it possible to draw a strong conclusion that drinking causes higher earnings on the basis of such a study?