We have seen this network in several guises now: Tea Scones, Disease Test, or, more generally, Hypothesis Evidence. Unlike the causal diagrams we will deal with throughout the book, a Bayesian network carries no assumption that the arrow has any causal meaning. The arrow merely signifies that we know the “forward” probability, P(scones | tea) or P(test | disease). Bayes’s rule tells us how to reverse the procedure, specifically by multiplying the prior probability by a likelihood ratio.
