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# Earned run average

{{sabermetric | formula = $mathrm\left\{ERA\right\} = 9 cdot frac\left\{mathrm\left\{Earned~Runs~Allowed\right\}\right\}\left\{mathrm\left\{Innings~Pitched\right\}\right\}$}}

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.

## 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 their win-loss record was considered sufficient in determining their effectiveness.

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.

## ERA and baseball era

As with batting average, the threshold 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 very good, although pitchers such as Greg Maddux and Pedro Martínez 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 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:

ERA Meaning
<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 for starters vs. relievers

It can be very misleading to judge relief pitchers solely on ERA, because they are charged only for runs scored by batters who reached base while batting against them. Thus, if a relief pitcher enters the game with his team leading by 1 run, 2 outs in the inning, and the bases loaded, then gives up a single which scores 2 runs, he is not charged with those runs. If he retires the next batter, his ERA for that game will be 0.00 despite having surrendered the lead. (He is likely recorded with a blown save in this situation.) Starting pitchers operate under the same rules but are almost never called upon to start pitching with runners already on base. In addition, relief pitchers know beforehand that they will only be pitching for a relatively short while, allowing them to throw each pitch with maximum energy, unlike starters who typically need to keep something in reserve in case they are called upon to pitch 7 or more innings. This freedom to use their maximum energy for a few innings, or even for just a few batters, helps relievers keep their ERAs down.

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.

## Sabermetric treatment of ERA

In modern baseball, Sabermetrics uses several defense independent pitching statistics including a defense-Independent ERA in an attempt to measure a pitcher's ability regardless of factors outside his control. Further, because of the dependence of ERA on factors over which a pitcher has little control, forecasting future ERAs on the basis of the past ERAs of a given pitcher is not very reliable and can be improved if analysts rely on other performance indicators such as strike out rates and walk rates. For example, this is the premise of Nate Silver's forecasts of ERAs using his PECOTA system. Silver also developed a "quick" earned run average (QuikERA or QERA) to calculate an ERA from peripheral statistics including strikeouts, walks, and groundball percentage. Unlike Peripheral ERA, it does not take into account park effects.

Rank ERA Player Team(s) Year(s)
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