Decision making is the physician's major activity. Every day, in doctors' offices throughout the world, patients describe their symptoms and com plaints while doctors perform examinations, order tests, and, on the basis of these data, decide what is wrong and what should be done. Although the process may appear routine-even to the physicians in volved-each step in the sequence requires skilled clinical judgment. Physicians must decide: which symptoms are important, whether any laboratory tests should be done, how the various items of clinical data should be combined, and, finally, which of several treatments (including doing nothing) is indicated. Although much of the information used in clinical decision making is objective, the physician's values (a belief that pain relief is more important than potential addiction to pain-killing drugs, for example) and subjectivity are as much a part of the clinical process as the objective findings of laboratory tests. In recent years, both physicians and psychologists have come to realize that patient management decisions are not only subjective but also prob abilistic (although this is not always acknowledged overtly). When doc tors argue that an operation is fairly safe because it has a mortality rate of only 1 %, they are at least implicitly admitting that the outcome of their decision is based on probability.