Gary, a psychologist, conducts a study to determine the relationship between beer sales and the number of road accidents (he suspects that selling more beer directly causes an increase in the number of accidents, because intoxicated drivers make mistakes!). He goes through his state's daily consumer sales and traffic reports for the past ten years, and he finds that an increase in beer sales is in fact correlated with an increase in the number of road accidents! However, it occurs to him that he still does not know if the relatinoship is causal - time of year could impact both the amount of beer sold (more beer is sold in the hot summer!) and it could simultaneously impact the number of road accidents (more road trips are taken in the summer - more cars on the road!). Perhaps increasing beer sales would not increase road accidents. In considering the unknown potential impact of the season on both beer sales and road accidents, what is Gary struggling with