Overview
Run expectancy quantifies the average runs expected to score from a specific base-out state through the end of an inning, calculated from historical data. For example, a runner on second with no outs carries a higher run expectancy than a runner on first with no outs, and the value grows the further the runners have advanced. This metric is essential for evaluating tactical decisions like stolen bases and sacrifice bunts. The break-even success rate for a stolen base derives from the change in run expectancy between a successful and a failed attempt. For sacrifice bunts, the benefit of advancing a runner is weighed against the cost of the out through the change in run expectancy, which has put their effectiveness as a run-scoring tactic under debate.