1.1 Prognosis and Prediction in Medicine Prognosis is central to medicine. All diagnostic and therapeutic actions aim to improve prognosis: • Screening : If we screen for early signs of disease, we may, for example, find cancers early in their course of disease, and treat them better than when they were detected later. But whether screening is useful depends on the improve- ment in prognosis that is achieved compared to a no screening strategy. Some cancers may not have caused any impact on life expectancy, while side-effects of treatment may be substantial. • Diagnosis : If we do a diagnostic test, we may detect an underlying disease. But some diseases are not treatable, or the natural course might be very similar to what is achieved with treatment. • Therapy : New treatments become available nearly every day, but their impact on prognosis is often rather limited, despite high hopes at early stages. “Magic bul- lets” are rare. Treatment effects are often small relative to the effects of determi- nants of the natural history of a disease, such as the patient’s age. The individual benefits need to exceed any side effects, harms and economic costs.