A transport office notes that its northern depot recorded 18 accidents last year while its southern depot recorded 9, and concludes that the drivers of the northern depot are the less careful of the two. In setting the two figures side by side, the office takes for granted which of the following?

Two bare counts can be set against each other only if they come from the same amount of exposure. If the northern buses run three times the distance, 18 accidents would show better care than 9, not worse, and the office's verdict reverses. Prompt entry in the register, the number of drivers on the roll and the fleets involved would each leave that gap between the two figures exactly where it is.