In baseball, earned run average (ERA) is a statistic used to evaluate pitchers, calculated as the mean of earned runs given up by a pitcher per nine innings pitched. A pitcher is assessed an earned run for each run scored by a baserunner who reached base while batting against that pitcher, whether by hit, base on balls or "walk", or being hit by a pitched ball; an earned run can be charged after the pitcher is relieved if he allows the runner before leaving the game. Runs scored by players who reach base on errors, passed balls, or catcher interference under special circumstances are treated as unearned runs, and do not count towards the pitcher's ERA.