There are no more motorists in Greatport than in Fairmont. Fairmont and Greatport are the cities with the highest collision-damage insurance rates. The insurance companies were already aware of the difference in the likelihood of collisions before the publication of the police reports. Repairing typical collision damage does not cost more in Greatport than in Fairmont.
For similar cars and drivers, automobile insurance for collision damage has always cost more in Greatport than in Fairmont. Police studies, however, show that cars owned by Greatport residents are, on average, slightly less likely to be involved in a collision than cars in Fairmont. Clearly, ttherefore, insurance companies are making a greater profit on collision-damage insurance in Greatport than in Fairmont.
Which of the following is an assumption on which the argument depends?