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Reversal is one of three elementary operations in evaluating well-formed (partial) relevance diagrams. A conditioning arrow between chance nodes A and B can be reversed when both nodes have an identical set of direct predecessors (except for A’s arrow into B). Informational arrows and conditioning arrows from a decision node cannot be reversed. Mathematically, reversing an arrow from chance node A to chance node B corresponds to applying Bayes’ theorem to their associated probability distributions. The direction of the arrow from A to B is flipped following a reversal operation. The resulting relevance diagram is decision-equivalent to the original and represents the same joint probability distribution over its uncertain variables.

See also: Addition, Direct Predecessor, Generalised Reversal, Removal, Well-Formed Relevance Diagram and Well-Formed Partial Relevance Diagram.