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Earned run average

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Template:Sabermetric

In baseball statistics, earned run average (ERA) is the mean of earned runs given up by a pitcher per nine innings pitched. It is determined by multiplying the number of earned runs allowed by nine and dividing by the number of innings pitched.

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 the 1900s—and, in fact, for many years afterward— pitchers were routinely expected to pitch a complete game, and their won-loss record was considered sufficient in determining their effectiveness. After pitchers like Otis Crandall and Charlie Hall made names for themselves as relief specialists, gauging a pitcher's effectiveness became more difficult using the traditional method of tabulating wins and losses. The National League first kept official earned run average statistics in 1912 (the statistic was called Heydler's Statistic for a while, after then-NL secretary John Heydler), with the American League following suit afterward.

Modern-day baseball encyclopedias notate ERAs for earlier years, but these were computed many years after the actual accomplishments. Negro League pitchers are often rated by RA, or total runs allowed, since the statistics available for Negro League games did not always distinguish between earned and unearned runs.

As with batting average, the value of a good ERA varies from year to year. In the 1910s, a good ERA was below 2.00 (two earned runs allowed per nine innings). In the late 1920s and 1930s, when conditions of the game changed in a way that strongly favored hitters, a good ERA was below 4.00; only a pitcher of the caliber of Dazzy Vance or Lefty Grove would consistently post an ERA under 3.00 during those years. In the 1960s, sub-2.00 ERAs returned as ballparks with different dimensions were introduced, among other influences. Today, an ERA under 4.00 is again considered exceptional, although pitchers such as Pedro Martinez and Greg Maddux stand out as Grove and Vance did in their day.

The all-time single-season record for lowest ERA in a season is 0.86, set by Tim Keefe in 1880. The modern record is 1.12, set by Bob Gibson in 1968. The lowest single-season ERA of an active pitcher is 1.56, achieved by Maddux in 1994. The career record is 1.82, held by Ed Walsh, and the active player with the lowest career ERA is Martinez, with an ERA of 2.71 through the 2004 season. The only other active non-relief pitcher with a career ERA under 3.00 through the 2004 season is Maddux at 2.95.

In modern baseball, an ERA under 2.00 is considered exceptional and is rare. An ERA between 2.00 and 3.00 is also considered excellent and is only achieved by the best pitchers in the league. An ERA between 3.00 and 4.00 is above-average. An ERA between 4.00 and 5.00 is average; the majority of pitchers have an ERA in this range. An ERA above 5.00 is generally considered below-average, and a pitcher with an ERA above 6.00 for a prolonged period of time is usually in danger of demotion to the bullpen or a lower league.

It can be misleading to judge relief pitchers solely on their ERA, because a pitcher is responsible only for the runs scored by batters who reach base off him. If a relief pitcher enters the game with his team leading by one run, 2 outs in the inning, and the bases loaded, then gives up a single which scores two runs, he is not charged with those runs. If he retires the next batter, his ERA for that game would be 0.00 despite having surrendered the lead.

ERA, taken by itself, can also be misleading for starting pitchers, though not to the extent seen with relief pitchers. ERA is affected to some degree by the park in which a pitcher's team plays half its games. For an extreme example, pitchers for the Colorado Rockies face a double problem. The high altitude of Denver causes fly balls to travel up to 10% farther than at sea level and reduces the ability of pitchers to throw effective breaking balls. Also, Coors Field has fences that are not long enough to compensate for the increased fly-ball distance at Denver, plus a relatively small amount of foul territory.ja:防御率 zh:防禦率

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