From BR Bullpen
An earned run is a run that is ruled by the official scorer to have resulted exclusively from actions by the batting team and not because of errors by the defense. When the fielding team makes an error or passed ball, the scorer is supposed to "reconstruct" the inning assuming that the defensive miscue did not occur. Only runs that would have scored barring the miscue are considered earned; all others are considered to be unearned.
- Example 1:
- Play: Batter A singles. While batter B is at bat, A advances to second on a passed ball. Batter B then hits a home run. Batter C reaches second base on a two-base error. Batter D singles C home. Batter E grounds out, with D advancing to second. Batter F doubles D home. Batter G grounds out, with F advancing to third. Batter H singles home F. Batter I strikes out to end the inning. 5 runs scored.
- Reconstructed inning: A singles. B hits a home run, scoring A. Batter C is out. Batter D singles, advances to second on E's groundout, and scores on F's double. G grounds out to end the inning. 3 runs scored.
- Because A, B, and D all score in the reconstructed inning, their runs are considered to be earned. C and F do not score in the reconstructed inning, so their runs are considered to be unearned.
The example shows two general rules of scoring earned and unearned runs:
- A batter who reaches base on an error can never score an earned run.
- A run that scores after there should be three outs is never earned.
The accounting of earned runs is more complicated when relief pitchers are used. Each pitcher is liable for the runners he allowed on base via hit, walk, or hit by pitch even after he is pulled for a reliever. Batters who replace a previous runner on a fielder's choice are charged to the previous pitcher. Also, when considering when the inning would be over except for errors, relief pitchers are not relieved of responsibility by errors that were committed before they were brought into the game. This means that some runs may be considered earned for an individual relief pitcher but not for the team as a whole, so team earned runs are often less than the sum of the earned runs allowed by the individual pitchers.
- Example 2:
- Play: Pitcher 1 is pitching. Batter A reaches first on an error. Batter B doubles, scoring A. C reaches on a bunt single, with B advancing to third. Pitcher 2 relieves 2. D grounds into a fielder's choice, with B scoring and C out at second. E triples, scoring D. F grounds out, with E scoring. G strikes out. Four runs score.
- In this case, the run by A is unearned because he reached base on an error. The run by B is earned and charged to 1. The run by D is also earned. Because D replaced C on a fielder's choice and C was originally allowed on base by 1, his run is charged to 1. The run by E is considered unearned for the team because the groundout by F should have ended the inning, but it's considered earned for Pitcher 2 because he doesn't get the benefit of the error committed while 1 was pitching.
Earned runs were originally intended as an offensive measure, showing how much of a team's scoring was the result of their batting success rather than the other team's fielding mistakes. Today is it used as a measure of pitching, showing how many of the runs given up by a pitcher were the result of his own mistakes rather than defensive failures.
See also: Earned Run Average