A failure of the accident severity classification.

This paper shows that procedures such as classification by accident severity will fail in their original purpose if (and since) accident severity is thought to be a continuous variable and not a step function- In the first place, accident severity within a severity class will be different between any two groups white basically have a different severity distribution: secondly, relation of severity on the risk of DF-injuries, say, will be distorted. General formulae are shown, as well as a set of example calculations.