Statistical Aspects of Injury Severity Part I: Comparison of Two Populations When There Are Several Grades of Injury

A method is proposed of comparing the average levels of injury sustained in two conditions. It consists of plotting the proportion of cases in one population that exceed an arbitrary threshold of severity against the proportion of cases in the other population that exceed the same threshold, as the threshold varies. The further this curve is from the 45 degree line, the greater is the difference in severity between the populations. The precise shape of the curve depends on the particular model considered: two examples are derived from considering the distributions of injury severity, respectively, each to be (i) normally distributed with the same variance, or (ii) exponentially distributed; the means of the distributions differing. A number of applications of this type of theory are outlined.