Using models to predict the future: what to do when the data run out?

Although clinical trials often provide "best evidence" comparing the effectiveness of alternative management strategies, such evidence can be limited in duration or in the results reported, causing clinicians and policy analysts to wonder "what if?" Models of the clinical prognosis-often spanning patients' lifetimes (the "long haul")-are perhaps weaker evidence, but can help answer questions about the management of individual patients and place that best evidence into the context of clinical reality.