An earned run in baseball is a run that scores against a pitcher without the benefit of an error or a passed ball by the defensive team. It represents a run that the pitcher is personally responsible for, as it results directly from the offensive team's production without any defensive errors influencing the scoring. To determine if a run is earned, the official scorer reconstructs the inning as if no errors or passed balls occurred. If no errors or passed balls happen during the inning, all runs scored are automatically earned. If a pitcher leaves the game with runners on base, any earned runs scored by those runners count against that pitcher. Earned runs are a key statistic used to calculate a pitcher's Earned Run Average (ERA), which measures the average number of earned runs a pitcher allows per nine innings pitched. Unearned runs, in contrast, occur due to defensive mistakes and are not counted against the pitcher's ERA. In summary, an earned run is one that scores due to the actions of the batting team alone, without defensive miscues benefiting the run, holding the pitcher accountable for that run.