The medical literature and the lay press often report substantial changes in the incidence or mortality of different diseases, including accounts of both large improvements in survival and warnings of burgeoning epidemics. If true, such information would be crucial for policy makers, patients, clinicians, and researchers. However, these reports can often be due to what Feinstein et al1 dubbed the “Will Rogers phenomenon” (also known as stage migration), rather than real change. Feinstein and coauthors noted that, with increased awareness and more liberal diagnostic testing, patients with milder stages of cancer who were previously not identified were subsequently counted in an expanded denominator, resulting in an apparent increase in incidence, and because these new cases had less severe or less advanced disease, there was an apparent decrease in mortality. Teasing out real changes from the Will Rogers phenomenon requires an approach that is immune to changes in diagnosis over time. However, with increasing diagnostic testing, the risk of the Will Rogers phenomenon is substantial for some diseases and conditions.