Adjusted ERA+

Adjusted ERA+, often simply abbreviated to ERA+ or ERA plus, is a pitching statistic in baseball. It adjusts a pitcher's earned run average (ERA) according to the pitcher's ballpark (in case the ballpark favors batters or pitchers) and the ERA of the pitcher's league.

Baseball-Reference.com originally calculated it as

${\displaystyle ERA+=100*{lgERA \over ERA}*PF}$

Where ERA is the pitcher's ERA, lgERA is the average ERA of the league, and PF is the park factor of the pitcher in question.

Today, they calculate it as

${\displaystyle ERA+=100*(2-{ERA \over lgERA}*{1 \over PF})}$

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Wikipedia