 In order to evaluate medical outcomes quantitatively, they have to be measured. There are numerous reasons for aiming to quantify medical outcomes. We may want to compare two or more treatments with respect to their efficacy at relieving certain symptoms or their ability to prevent deaths due to a certain disease. When resources are scarce, we may not only want to invest in treatments that are efficacious (that is, they do improve patient morbidity, mortality or both) but also efficient (that is, it is more efficacious than other treatments relative to the cost of procuring it). For matters of international comparison, development and international justice, we also want to have measures of disease burden: Which of a number of tropical diseases has the highest cost in terms of increased morbidity and mortality? For each research dollar spent on treatments for disease X, how much can we expect to reduce the morbidity and mortality it causes?