Early references to the mutational origin of cancer.

Survival-related biases, potential or actual, affecting inferences on health and disease in the population of the selected survivors have a long history in epidemiology, at least since Ogle noted some aspects of the ‘healthy worker effect’. They reflect either the selective admission of survivors into a study or the selective availability of information only on survivors. These biases continue to creep in, often unrecognized, not only in clinical but, more surprisingly, in epidemiological studies. Three recent cohort studies, with potential important implications at practical level, illustrate the point.