A study in one state compared the traffic fatality rates (number of fatalities per miles driven) on highways with 55, 65, and 75 miles per hour speed limits. They found that highways with a 75 mph speed limit had the lowest fatality rate and highways with a 55 mph speed limit had the highest fatality rate. Traffic fatalities could evidently be reduced by raising speed limits, perhaps because people pay more attention when they are driving fast. What is the biggest statistical problem with this study?