An economic evaluation is designed to prioritize expenditure, aid decision making, and inform medical audit. A good economic evaluation is explicit and clearly describes the alternative therapies to be evaluated. The evaluation takes into consideration the cost of each component and measures outcome in economic benefit and physical, social, and psychologic well-being. A variety of approaches can be used to assess quality of life, and a good economic evaluation will use a range of measures. Costs and outcomes of health care interventions typically accrue across time periods, and the analysis must allow for this. Similarly, data must answer the question; how much does it cost to identify one additional unit of benefit? Because all evaluations are imprecise both in cost and outcome, sensitivity analysis is needed to determine the effect that different assumptions have on the results. © 1990.