[The Will Rogers phenomenon and its impact on imaging diagnostics]

The Will Rogers phenomenon is a possible cause of systematic distortions in the results of clinical studies, which can be produced if stage migration occurs during a disease. The term refers to the apparent paradox which is observed when an element is changed from one set to another and the average values of both sets are altered in the same way. The effect is due to the prerequisite that the numerical value of the element being moved is placed between the mean values of both groups. In medicine, this phenomenon is a consequence of the evolution of staging procedures and may be source of misleading statistics for survival in cancer. Both advanced pathological assessment and modern imaging techniques may be involved. The wrong conclusions are induced by comparing the effects of treatment in contemporary patient groups, which profit from extensive diagnostic procedures, to those of historical controls. Treatise informs about the history of the term and illustrates its effects by numerical examples and clinical data. Finally, a model computation based on current PET/CT figures is offered.