patients in the clinical arena. It differs, however, in that most clinical rules are applied prospectively to predict the likelihood of an outcome or event whereas the PC is applied retrospectively to analyze the likelihood that a known cancer was caused by radiation. In that respect it is akin to the statistical concept of attributable risk. In other words, clinical-prediction rules are conditioned on a set of factors and a prior probability to predict a future event whereas the PC is conditioned on the event’s having already occurred. In some sense, clinical predictions are a priori chances, whereas PCs are a posteriori chances. Both are probabilities, and both range from 0 to 1. In recent years, when these analytic likelihoods (PCs) have been applied to individuals, they have been called the assigned share (AS) of the risk, and the sum of all such assignments (base plus excess) equals unity. The PC/AS concept is developed in detail in Chapter 5.