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 surrender over the course of a full game had he been kept in for the full nine innings. It is determined by dividing the number of earned runs allowed by the number of innings pitched and multiplying by nine. Unearned runs, resulting from batters 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.
Since pitchers have sole responsibility to earn strikes against opposing batter(s) until at least three batters are retired (struck out) in each inning of play, and nine innings (a complete game) are pitched and; they are assessed an earned run when each batter/runner scores a run; some means had to be found to calculate the apportionment of earned run responsibility where multiple pitchers assume responsibility in a single game. After pitchers like James 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.
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 0.96, set by Dutch Leonard in 1914. The lowest single-season ERA of a pitcher since 1950 is 1.12, achieved by Bob Gibson in 1968. The career record is 1.82, held by Ed Walsh, and the active player with the lowest career ERA (among those with more than 1,000 innings pitched) is Mariano Rivera, with an ERA of 2.30 through the 2008 season. Rivera, though he is a relief pitcher, has more than 1,000 lifetime innings pitched, earning the right, in many fans' minds, to be considered on an equal footing with starters in debates involving the term "greatest pitcher".
Some sources may list players with undefined or infinite career ERAs. This can happen if a pitcher allows one or more earned runs without retiring a batter (usually in a single appearance). An undefined ERA occasionally occurs at the beginning of a baseball season. It is sometimes incorrectly displayed as zero or as the lowest ranking ERA when it is more akin to the highest.
In modern baseball, ERAs can be interpreted in the following way:
|<2.00||Considered exceptional and is rare.|
|2.00 to 3.00||Excellent, only achieved by best pitchers in the league.|
|3.00 to 4.00||Better than average.|
|4.00 to 5.00||Average.|
|5.00 to 6.00||Worse than average.|
|>6.00||Consistently having an ERA this high risks demotion to the bullpen, or a lower league.|
ERA, taken by itself, can also be misleading when trying to objectively judge starting pitchers, though not to the extent seen with relief pitchers. The advent of the designated hitter rule in the American League in 1973 made the pitching environment significantly different— pitchers spending all or most of their careers in the AL have been at a disadvantage in maintaining low ERAs compared to National League pitchers who can often get an easy out facing the opposition's ninth batter (oddly, Martinez and Rivera, the ERA kings of the last decade or so, have been mostly active in the American League).
This discrepancy between the leagues also affects relievers, but not to the same degree, as National League relievers actually pitch to pitchers far less than do National League starters for a number of reasons, chiefly because relievers are usually active in later innings when pinch hitters tend to be used in the ninth spot. ERA is also affected somewhat by the park in which a pitcher's team plays half its games, as well as the tendencies of hometown official scorers to assign errors or base hits in plays that could be either.
For an extreme example, pitchers for the Colorado Rockies have historically faced many problems, all damaging to their ERAs. The combination of high altitude and a semi-arid climate found in Denver causes fly balls to travel up to 10% farther than at sea level. Denver's altitude and low humidity also reduce the ability of pitchers to throw effective breaking balls, due to both reduced air resistance and to difficulty in gripping very dry baseballs. Also, the fences at Coors Field are not far enough from home plate to compensate for the increased fly-ball distance. The field also has a relatively small amount of foul territory. These conditions have been countered to some extent since 2002 by the team's use of humidors to store baseballs before games. These difficult circumstances for Rockies pitchers may not adversely affect their won-lost records, since opposing pitchers must deal with the same problems. Indeed, hometown hurlers have some advantage in any given game since they are physically acclimated to the altitude and often develop techniques to mitigate the challenges of this ballpark. Still, conditions there tend to inflate Rockie ERAs relative to the rest of the league.
|1||1.82||Ed Walsh||Chicago (AL), Boston (NL)||1904–17|
|2||1.89||Addie Joss||Cleveland (NL)||1902–10|
|3||1.89||Jim Devlin||Chicago (NA), Louisville (NL)||1875-77|
|4||2.02||Jack Pfiester||Pittsburgh (NL), Chicago (NL)||1903-04, 1906-11|
|5||2.03||Smoky Joe Wood||Boston (AL), Cleveland (AL)||1908-15, 1917-22|