Earned run average

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In baseball statistics, earned run average (ERA) is the average of earned runs allowed by a pitcher per nine innings pitched (i.e. the traditional length of a game). It is determined by dividing the number of earned runs allowed by the number of innings pitched and multiplying by nine. Thus, a lower ERA is better. Runs resulting from passed balls, defensive errors (including pitchers' defensive errors), and runners placed on base at the start of extra innings are recorded as unearned runs and omitted from ERA calculations.

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