An auto-parts store offers a fuel additive that claims to increase a vehicle’s gas mileage. The additive is poured into a vehicle’s gasoline tank after the tank is filled. To measure the claim, the store offers a free bottle of the additive to customers who drive SUV's, who then provide information about their gas mileage on the store website. A customer argues that the type of vehicle could be a confounding variable in this study.
Answer
A
Because vehicles of different sizes may respond similarly to the additive, it should be easy to conclude whether the additive improves gas mileage.
B
Because vehicles of different colors may show differences in gas mileage, the additive may improve the gas mileage in some vehicles but not in others.
C
Because owners of different types of vehicles may choose not to use the additive, those vehicles will not show an improvement in gas mileage.
D
Because vehicles of different sizes may respond differently to the additive, it may be difficult to conclude whether the additive improves gas mileage for all vehicles.