First and foremost, what is evidence? The traditional research paradigm values information on the basis of its internal validity, or freedom from the potential for bias and inferential error (5). However, patients, providers, and policy makers largely understand evidence in terms of its utility. As defined in the American Heritage Dictionary, evidence is a "thing or things helpful in forming a conclusion or judgment." Users of evidence typically must make decisions in the face of high uncertainty, extreme time constraints, and competing demands. Under these imperfect conditions, they seek evidence—perfect or otherwise—to help them make the best choices.