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. Runs resulting
from batters who reach base on an error (even a pitcher error) and
later score are called unearned runs, and do not count toward
ERA.
Origins
{{sabermetric | formula = \mathrm{ERA} = 9 \cdot
\frac{\mathrm{Earned~Runs~Allowed}}{\mathrm{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 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.
Some means had to be found to calculate the apportionment of
earned-run responsibility where multiple pitchers assume
responsibility in a single game since pitchers have sole
responsibility to earn strikes against opposing batters. A pitcher
is assessed an
earned run for each earned run
scored by a batter (or pinch-runner) who reached base while batting
against that pitcher. 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 in different decades and baseball eras
As with
batting average, the
definition 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 high-caliber pitchers, for example
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 other influences
such as ballparks with different dimensions were introduced. Today,
an ERA under 4.00 is again considered good, with pitchers such as
Greg Maddux and
Pedro Martínez achieving this
mark.
The all-time single-season record for lowest ERA 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 (1904-17). The active player with the
lowest career ERA (among those with more than 1,000 innings
pitched) is
Mariano Rivera, with an
ERA of 2.29 through the
2008
season.
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). Additionally, 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, even though 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: starters vs. relievers; DH rule; ballpark; altitude &
climate
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, with
2 outs and the bases loaded, and then gives up a single which
scores 2 runs, he is not charged with those runs. If he retires the
next batter (and pitches no more innings), 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 asked 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. Since then,
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 when pitching to the
opposition's pitcher, who is usually not a very good batter.
(Interestingly, though,
Pedro
Martinez and
Mariano Rivera, the
ERA kings of the last decade or so, have been mostly active in the
American League.) Since 1997, when teams began playing teams from
the other league during the regular season, the DH rule is in
effect only when such
interleague games are played
in an American League park.
This difference between the leagues (the DH) also affects
relievers, but not to the same degree, as National
League relievers actually pitch to pitchers far less than do NL
starters for a number of reasons, chiefly because relievers are
usually active in later innings when
pinch
hitters tend to be used in the pitcher's batting spot.
ERA is also affected somewhat by the
ballpark in
which a pitcher's team plays half its games, as well as the
tendency of hometown official scorers to assign errors instead of
base hits in plays that could be either.
As an extreme example, pitchers for the
Colorado Rockies have historically faced
many problems, all damaging to their ERAs.
The combination of
high altitude (5,280 ft. or 1,609 m.)
and a semi-arid climate 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 difficulty in gripping very dry baseballs. 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 OR PERA, it does not take into account
park effects.
All-time career leaders
See also
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