Evaluating validity Evaluating validity Laupacis and colleagues of the Evidence-Based Medicine Working Group have described several important questions regarding validity of an article about prognosis. Many studies are done in referral settings, where the sickest patients end up. The study may describe the prognosis of more severely ill patients, rather than the patients typically seen in the primary care setting. In a study about breast cancer prognosis, patients should be at a similar stage in the disease. Otherwise, it is difficult to interpret the results.
Mortality is a clearly defined outcome.
Some studies, though, use more subjective outcomes. For example, a study of the decline in patients with Alzheimer's may have institutionalization as a primary outcome . Clearly, the decision to institutionalize is social and behavioral as well as medical. It is important that patients are followed long enough for the outcome in question to occur. Five years is a minimum for most chronic illnesses, and ten years is preferable.