The Importance of Class Probability

Case probability means we know, with regard to a particular event, some of the factors which determine its outcome. But there are other determining factors about which we know nothing. All medical prognoses, when based only on physiological knowledge, deal with class probability. A surgeon tells a patient who considers submitting himself to an operation that 30 out of every hundred undergoing such an operation die. If the patient asks whether this number of deaths is already full, he has misunderstood the sense of the doctor's statement. He has fallen prey to the error known as the gambler's fallacy.