Bayes’s theorem is an extremely powerful means of using information at hand to estimate probabilities of outcomes related to the occurrence of preceding events. Bayes' Theorem uses a priori (subjective) and conditional probabilities to calculate the probability of an uncertain event occurring. A priori probabilities represent what the modeler believes, before testing, to be the probability of an event occurring. Conditional probabilities are probabilities that other events occur in conjunction with the original event.