Adjusted ERA+
What is Adjusted ERA+?
A pitching statistic in baseball that is an adjustment of a pitcher's ERA, based on his home ballpark (whether it favors pitchers or batters) and also the average ERA of the particular league in which he pitches. An average adjusted ERA+ is 100, with a score over 100 indicating that a pitcher has performed better than the average, and a score below 100 pointing to below-average performance.
Also referred to as "ERA Plus".
Sporting Charts explains Adjusted ERA+
For example, if the league-average ERA is 4.00 and the pitcher pitches in a hitter-friendly park, and the pitcher's ERA is right at 4.00, then his adjusted ERA+ will be in excess of 100. Conversely, if the league-average ERA is 3.00, and he pitches in a pitcher-friendly park, and his ERA is right at 3.00, then his adjusted ERA+ will be less than 100. This makes it possible to compare pitchers across different leagues. In the given example, the pitcher with the higher ERA actually performed better than the pitcher with the lower ERA.
The single-season record for ERA+ is held by Pedro Martinez, as he posted an ERA+ of 291 in 2000. The career leader in adjusted ERA+ is currently Mariano Rivera.