Clinicians spend their working lives making decisions. such decisions are usually made in interlocking streams rather than in the discrete circumscribed contexts so beloved of scientists. When the clinician encounters a patient a complex interactive process is initiated in which the clinician searches his memory to match the symptoms and signs indicated by the patient with the complex disease models which he carries in his head. He then makes choices about further questions or tests in order to clarify his understanding of the patient's problem and to formulate a management or treatment plan....