In baseball statistics, earned run average (ERA) is the mean of earned runs given up by a pitcher per nine innings pitched (i.e. the traditional length of a game). It is determined by dividing the number of earned runs allowed by the number of innings pitched and multiplying by nine. Runs resulting from defensive errors (including pitchers' defensive errors) are recorded as unearned runs and are not used to determine ERA.

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.

Some criterion was needed to capture the apportionment of earned-run responsibility for a pitcher in games that saw contributions from other pitchers for the same team. Since pitchers have primary responsibility to put opposing batters out, they must assume responsibility when a batter they do not retire at the plate moves to base, and eventually reaches home, scoring a run. 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.

In baseball, an earned run is any run that was not necessarily enabled by a fielding error or a passed ball. In other words, an unearned run is one run that would not have been scored without the aid of an error or a passed ball committed by the defense, and an earned run is simply one that is not unearned. An error made by the pitcher in fielding at his position is counted the same as an error by any other player.

Regardless of the name, an unearned run is credited to the offensive team's score the same as any other run. It is only "unearned" in that it was, in a sense, "given away" by the defensive team.

Both total runs and earned runs are tabulated as part of a pitcher's statistics, but earned runs are specially denoted because of their use in calculating a pitcher's earned run average (ERA). One of the most prominent of baseball's traditional statistics for pitchers, the ERA is defined as the number of earned runs allowed per 9 innings pitched (i.e., averaged over a regulation game).

In baseball statistics, run average (RA) refers to measures of the rate at which runs are allowed or scored. For pitchers, the run average is the number of runs—earned or unearned—allowed per nine innings. It is calculated using this formula:

where

Run average for pitchers differs from the more commonly used earned run average (ERA) by adding unearned runs to the numerator. This measure is also known as total run average (TRA) or runs allowed average. For batters, the run average is the number of runs scored per at bat.

Run average for pitchers

Although presentations of pitching statistics generally feature the ERA rather than the RA, the latter statistic is notable for both historical and analytical reasons. For early leagues or leagues for which statistics must be calculated from box scores, such as the Negro leagues, data on earned runs may be unavailable and RA may be the only statistic available. The analytical case for RA appeared as early as 1976, when sportswriter Leonard Koppett proposed that RA would be a better measure of pitcher performance than ERA. Subsequently, sabermetrician Bill James wrote, "I think that the distinction between earned runs and unearned runs is silly and artificial, a distinction having no meaning except in the eyes of some guy up in the press box."