Natural” experiment? A recent study compared the rate of crashes involving 16- to 18-year-old drivers in two adjacent Virginia counties. For two consecutive years, there were more crashes per person for drivers in this age group in Chesterfield County, where school starts at 7:20 A.M. , than in Henrico County, where school starts at 8:45 A.M . The study was described as a “natural experiment,”

because the two counties are nearly identical in socioeconomic characteristics and in the percentage of roads with traffic congestion. Explain why it is harder to establish a cause-and-effect conclusion with this type of

“natural experiment” than an experiment that uses random assignment.