Standardized comparison of performance indicators in trauma: a new approach to case-mix variation.

An institution's trauma survival rate can be compared with that predicted by TRISS using definitive outcome-based evaluation. This examines W, the difference between actual and predicted survival rates; Z, the statistical significance of this difference; and M, a measure of the similarity of injury severity mix to the prediction data base. However, it is possible for two institutions with the same survival rate within each band of injury severity to have very different W and Z scores whilst retaining a similar M score. Clearly this is unsatisfactory. A new statistic, Ws, is therefore proposed, which is standardized with respect to injury severity mix, producing more accurate comparisons between different institutions. Confidence intervals are used to graphically illustrate the magnitude of Ws, its direction, accuracy, and statistical significance. Data from the U.K. Major Trauma Outcome Study are used to demonstrate the calculations and presentation of Ws and its advantages.