reference class problem: in trying to determine the outcome probability of a particular experiment, the frequentist has to place it in a reference class of “similar” experiments with known outcome frequencies. I. J. Good (1983, p. 27) wrote, “every event in life is unique, and every real-life probability that we estimate in practice is that of an event that has never occurred before.” For example, given a particular patient, a frequentist who wants to estimate the probability of a cavity will consider a reference class of other patients who are similar in important ways—age, symptoms, diet—and see what proportion of them had a cavity. If the dentist considers everything that is known about the patient—weight to the nearest gram, hair color, mother’s maiden name—then the reference class becomes empty. This has been a vexing problem in the philosophy of science.