Evaluation of trauma care must be an integral part of any system designed for care of seriously injured patients. However, outcome review should offer comparability to national standards or norms. The TRISS method offers a standard approach for evaluating outcome of trauma care. Anatomic, physiologic, and age characteristics are used to quantify probability of survival as related to severity of injury. TRISS offers a means of case identification for quality assurance review on a local basis, as well as a means of comparison of outcome for different populations of trauma patients. Methods for calculating statistics associated with TRISS are presented. The Z and M statistics are explained with the nonstatistician in mind. We feel this article is a source for those interested in developing or upgrading trauma care evaluation.