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 predic ted 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 s imilarity of injury severity mix to the prediction data base. However, it is possible for two institutions with the same survival rate withi n each band of injury severity to have very different W and Z scores w hilst retaining a similar M score. Clearly this is unsatisfactory. A n ew statistic, W-s, is therefore proposed, which is standardized with r espect to injury severity mix, producing more accurate comparisons bet ween different institutions. Confidence intervals are used to graphica lly illustrate the magnitude of W-s, its direction, accuracy, and stat istical significance. Data from the U.K. Major Trauma Outcome Study ar e used to demonstrate the calculations and presentation of W-s and its advantages.