The Three Faces of Probability One important fact is often overlooked. Probability is not one of a kind; it was born with three faces: frequency, physical design, and degrees of belief.10 And these have persisted to this day. Frequency. In the first of its identities, probability is about counting. Counting the number of days with rainfall or the number of hits a baseball player makes and dividing these by the total number of days or strikes results in probabilities that are relative frequencies. Their historical origins lie in seventeenth-century mortality tables, from which life insurances
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