}}} In baseball statistics, earned run average (ERA) is the mean of earned runs given up by a pitcher per nine innings pitched. The ERA tells the average number of runs a pitcher would inadvertently surrender over the course of a full game had he been kept in. It bears similar meaning to a hitter's batting average. It is determined by dividing the number of earned runs allowed by the number of innings pitched and multiplying by nine. Runners reaching base on errors (even errors by pitchers) do not count toward ERA if they later score. To a pitcher, a lower earned run average is preferable to a higher one. Origins Henry Chadwick is credited with first devising the statistic, which caught on as a measure of pitching effectiveness after relief pitching came into vogue in the 1900s. Prior to 1900 — and, in fact, for many years afterward — pitchers were routinely expected to pitch a complete game, and th