In the performance of probabilistic risk assessments, in which there are inevitably large uncertainties, it is customary to characterize the computed probabilities in terms of their medians. When this is done, it is incorrect to add the probabilities of different accident sequences to find an overall probability of some consequence (like core melt), or to add the risks of the members of a population of reactors to find the societal risk. The error is not only one in principle, but is substantial when the uncertainties are large. In addition, the uncertainties are reduced when the probabilities are combined properly. Some examples are given.