In any long-term study the doctor's experience is biased. The patients whom he sees are no fair sample of the sick population and those followed are biased by an overload of persons not doing well, who keep returning to him. So the percentage description of the medical experience, for many years the standard method of analyzing it, does not yield as valid information as has been generally believed. In this study, escape from these biases was sought by pairing the data and the method used could be applied to any medical experience. Two hundred twenty-one patients were followed for five years or longer after a high frequency force ballistocardiogram. After interfering factors had been eliminated, marked abnormalities of ballistocardiographic form were still associated with great reduction in longevity.
Starr I. Prognostic Value of BallistocardiogramsStudies on Evaluation of the Doctor's Experience. JAMA. 1964;187(7):511–517. doi:10.1001/jama.1964.03060200043008