Earned run average

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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 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.



\mathrm{ERA} = 9 \times \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 put opposing batters out. 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.

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