My colleague, Connor O’Brien, did a series of getting to know your sabermetrics statistics in 2016. I wanted to continue his efforts to bring more stats to you that many baseball front offices believe are important in player evaluation.

The one we begin with today is league adjusted ERA (ERA+). Of course ERA is one of the most important statistics when looking at a pitchers season. It’s great when looking at Clayton Kershaw vs. Max Scherzer in a single season. But how about a pitcher like Jake McGee who had to pitch at Coors field against a pitcher like Addison Reed.

League adjusted ERA or ERA+ is calculated as 100*[lgERA/ERA] on Baseball Reference. It adjusts to a player’s ballpark after that. A park factor of one would mean there’s no advantage or disadvantage by pitching in that stadium. A park factor lower than one means it is pitcher friendly and a park factor greater than one means it is hitter friendly.

In a more basic sense, we’re seeing how much better a pitcher was to the rest of league in terms of percentage. If a player posts a 210 ERA+, they’re 110% better than the rest of the league.

For example, say Noah Syndergaard posts a 2.50 ERA in a year where the league ERA is 4. Thor would have a raw ERA+ of 160 and it would adjust accordingly to the ball park he plays in. Citi Field isn’t a serious bandbox and it’s not a pitchers haven so his adjusted ERA would likely stay in that 160 range, give or take.

Let us take a look at some examples to further exemplify why this stat is important. Chris Rusin and Luis Garcia both posted a 2.65 ERA in 2017. Garcia was a Philly so his home park was Citizens Bank Park. Rusin was a Rockie so his home park was Coors field. Garcia’s ERA+ is 160 whereas Rusin’s is 189. Another way to think about this, Garcia’s ERA in Coors field would have been higher than 2.65. So this is one example of why using just ERA can be flawed, it doesn’t give enough credit to a player who pitches in an extremely tough stadium.

The next example is a fun one in my opinion and before I begin, I think Clayton Kershaw is the best pitcher of this generation, he’s a Hall of Famer, and his playoff struggles are irrelevant when disussing his greatness. Clayton Kershaw from 2011 to now, has posted a 2.1o ERA in 1,452 innings pitched. From 1997-2003, Pedro Martinez posted a 2.20 ERA in 1,408 innings pitched. Those numbers are nearly identical except Kershaw has 48 more innings in his six year span with a slightly better ERA. So Kershaw must be better because his ERA is better, right?

Absolutely no, Pedro pitched in the height of the steroid era in Fenway Park. Pedro as a result had a 213 ERA+ in his six year reign while Kershaw had a 179 ERA+ in his six year reign. As a result of this, Pedro’s WAR from 1997-2003 was 57.3 and Kershaw’s from 2011-2017 was 45.8. That’s almost a 12 win difference in almost the same number of innings.

Pedro posted a 291 ERA+ in 2000. He had a 1.74 ERA that year but in 2014, Kershaw had a nearly identical 1.77 ERA. Kershaw’s ERA+ was almost 100 points lower than Pedro’s amazingly enough though. That’s how amazing Pedro Martinez was in his era. He puts Clayton Kershaw to shame and that is not easy work.

My final example is between a Hall of Fame pitcher and someone who will likely be joining him soon.  Tom Glavine posted a 3.54 ERA for his 22-year career which spanned 4,413.1 innings. Mike Mussina posted a 3.68 ERA for his career which spanned 3562.2 innings. Despite the roughly 850 innings that Glavine threw that Mussina did not and the better ERA , Mussina beats him in WAR.

Why? Well, Mike Mussina pitched in the AL East bandboxes in the height of the steroid era. Glavine is an all-time great, but he mostly pitched in a large pitchers park for most of his career. Mussina owns a 123 ERA+ for his career while Glavine owns a 118 ERA+ for his. As a result, Glavine’s pitching WAR was 74 while Mussina’s was 82.7. The quality of innings that Mussina gave were just better and Mussina was every bit of a workhorse that Glavine was. Mussina averaged 198 innings pitched every year of his career while Glavine pitched 201 innings a year.

So there you have it, adjusted ERA gives players who played in a tough stadium or in a tougher era some more love.