ERA+: This is the pitching version of OPS+. As it is with a hitter’s Batting Average, ERA is a statistic that most baseball fans understand in that it represents “Earned Runs allowed per 9 innings.” So, if a pitcher has an ERA of 3.00, you would expect them to give up 3 earned runs in a 9 inning game. ERA+ adjusts the traditional ERA statistic for ballpark effects and the overall pitching environment in the era that each hurler toed the rubber. ERA+ is set so that 100 is league average. Therefore, if your ERA+ is over 100, you’re above average, it’s below 100, you’re below average.
The ballpark factors are obvious to understand: it’s easier to post a low ERA at Dodger Stadium then at Coors Field. But the friendliness of each baseball year to hitters or pitchers is relevant too. It was easier to post a low ERA in the dead ball era (pre 1918) or in the high-mound era (1962-1968) than the PED era, where dozens of hitters were fueled by steroids or other Performance Enhancing Drugs.
- A great apples to apples comparison can be seen with the great Dodgers lefties Sandy Koufax and Clayton Kershaw. In the last 6 years of his career (1961-1966), Koufax posted a 2.19 ERA (which came out to an ERA+ of 156). Fifty years later (from 2011-2016), Kershaw put up an ERA of 2.06 with an ERA+ of 178. So his ERA was slightly better but his ERA+ was significantly higher because it was tougher to get outs from 2011 to 2016 than from 1961-1966.
- An even more telling example would be to compare Koufax to Pedro Martinez at their peaks. Each Hall of Famer had a four-year stretch in which they won 3 Cy Young Awards (Koufax’s was from 1963-1966, Pedro’s from 1997-2000). Sandy had a 1.86 ERA for those years; Pedro’s ERA was 2.16 for his four best. So, without any adjustments, Koufax’s four years were better. But, when adjusted for the eras in which they toiled, Koufax’s ERA+ checks in at 172 (still incredibly great, 72% better than average). But Martinez’ four-year run reflected an ERA+ of a ridiculously high 219. Simply put, posting a 2.16 at the height of the PED era (and with three of those four seasons pitching in Fenway Park) is more impressive than posting a 1.86 ERA at Dodgers Stadium in the 1960’s.
- The limitation of ERA+ is the same as it is for ERA. The rules about earned runs sometimes unduly benefit or punish a pitcher. If a pitcher gets two outs and then the next batter reaches base on an error, all runs for the rest of that inning are considered unearned. So you could give up six home runs in a row after that error and all of those runs would be unearned. For this reason, some analysts like to use the stat RA9, which is calculated exactly like ERA except that all runs are counted.