Motor-scooter dealers attribute a drastic decline in sales over the last few years to a...

Motor-scooter dealers attribute a drastic decline in sales over the last few years to a new law requiring motor-scooter riders to wear helmets. Previously, helmets had been

obligatory for motorcycle riders but not for motor-scooter riders—a difference that the dealers argue made scooters preferable for many customers. Safety advocates,

however, dispute the dealers’ explanation, pointing out that the law’s introduction coincided with a large increase in the cost of mandatory insurance for both types of

vehicle.

In evaluating the safety advocates’ and the dealers’ explanations, it would be most helpful to know which of the following?