Earned Run Average (ERA) is a rudimentary metric designed to assess how well a pitcher has done in the past. To calculate, divide a pitcher’s total number of Earned Runs allowed by his total number of Innings Pitched and multiply by nine.
ERA = (Earned Runs / Innings Pitched) * 9
ERA is not a good predictor of future success, as Earned Runs are dependent on multiple factors outside of the pitchers control: defense, umpiring, the judgment of a scorekeeper, etc. Pitchers aren’t held accountable for runs scored as a result of an Error by one of their fielders, but they are held accountable for runs scored on bloop hits that get by slow or poor defenders. If a pitcher has a poor defense behind him, he will likely end up with a higher ERA than he should have.
As a result, a pitcher’s ERA is a poor estimate of their true talent level. Consider this scenario. In your mind, who is the better player: the pitcher that strikes out the side, or the pitcher that relies on his rangy defense to haul in three deep fly balls? ERA thinks that both pitchers are just as good, but that’s not telling you the full picture.
Please note that the following chart is meant as an estimate, and that league-average ERA varies widely on a year-by-year basis. To see the league-average ERA for every year from 1901 to the present, check the FanGraphs leaderboards.
Things to Remember:
● ERA is difficult to compare across teams due to differences in team defenses, difficult to compare across leagues due to competition imbalance and the DH, and difficult to compare across years because of different run-scoring environments. A 3.50 ERA has a different meaning depending on if that pitcher played in the early 2000s or the Dead Ball Era, or if they played in a pitcher’s or hitter’s park.
● To adjust for park and league effects, check out ERA-. It’s still not a perfect statistic, but it does make it easier to compare pitchers from different time periods and parks.
Links for Further Reading: