Classic principles of treatment in orthopaedic surgery—the immobilization of fractures or the draining of infected wounds, for example—were not first established in prospective clinical trials or laboratory experiments. Rather, they were derived from perceptive observation: the methods were seen to work in practice, and they were retained. Observation has a noble history in medicine and science. Still, the modern reader is at least intuitively aware of the limitations of mere observation. Imagine if an investigator were to claim that prophylaxis against deep-vein thrombosis after hip replacement is not needed simply because only two thromboses were observed in ten patients who did not receive prophylaxis compared with three thromboses in ten patients who did. Such a study, were it to be published, would be the object of ridicule.