Sabermetric of the Day: ERA+

Sabermetric of the Day

Let’s do some numbers stuff for the nerds! Today, we’re going to take a look at an interesting sabermetric called ERA+. It’s interesting. Yes, very interesting.

What is ERA+?

ERA+ stands for “adjusted ERA.” It is defined as “a pitchers earned run average adjusted for parks pitched in and the average ERA of the pitcher’s league.” Seem pretty straightforward? Good. Here, let me complicate it.

The original calculation for ERA+ was:

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

In the calculation (of both, actually), lgERA stand for the average league ERA. ERA is the normal ERA of the pitcher that you want to know the ERA+ for. And PF stands for “park factor” AKA a number that some geeks figured out to adjust for how each park in the Bigs plays differently.

Now, the calculation is:

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

Don’t ask me why they needed to flip stuff over and added a two. I’m not a mathematician. Either way, you get a number that tells you what the pitcher’s adjusted ERA is. For every year, a perfectly average ERA+ is 100. Anyone who threw better than average scores above 100 and anyone worse than average scores under 100. Kapeesh?

Okay but who cares?

Who cares?! Baseball cares! Let’s say the league average ERA is 4.00. Now, let’s say we have two pitchers, Abraham Lincoln and George Washington, who each threw to a lovely 3.50 ERA. According to the standard ERA formula, these two guys gave up the same amount of runs every time they threw 9 innings. Which is true, to be fair. Buuuut let’s say Abraham Lincoln’s home stadium was Colorado. Yeah, baby, he’s throwing half his starts a mile high. Now, let’s say George Washington is a San Francisco Giant so he throws in the graveyard that is AT&T every other time out. Now things look a little different, don’t they? They have the same ERAs, sure, but Lincoln’s ERA+ will be well above 100 while Washington’s will probably be under 100.

Sure…but how does that affect anything?

How does that affect anything?! Child, please! Let’s keep saying that the league’s average ERA is 4.00. And, this offseason, the Chitown Cubbies are looking to pick up a third starter. Lucky for them, Thomas Jefferson is on the market and he posted a 3.00 ERA this year for the San Diego Padres! Uh oh, that means he’ll be expensive…But…Wait a minute…He played in Petco for half his games! His ERA+ could be way less than average when you take that into accont! Well, Theodore Roosevelt is also on the market and he put up a 4.50 ERA as a Ranger throwing half his games in Arlington. Bet you his ERA+ is better than Jefferson’s – and he’ll be cheaper. (Of course, he’ll only be cheaper if the rest of the league doesn’t catch onto the ERA+ thing but you get the picture.)

Oh, now I get it. Stop naming presidents and give us some real examples.

Well, okay then, smartypantses. Here are the top three pitchers of all time using adjusted ERA:

1. Mariano Rivera

Mariano Rivera 

Yeah, you better believe it, bud. The prolific 19 year career that the Yankees’ closer put together was something to behold. His ERA+ was 205 over the course of his entire career! 205! That means for 19 years, he was two times better than the average Major Leaguer. That’s insane! The dude is an absolute legend but that is otherworldly.

2. Clayton Kershaw

Clayton Kershaw

Mind as well go from the best pitcher of all time to the best pitcher of right now, right? His 161 ERA+ is unbelievable, just as you would expect it to be seeing his regular ERA numbers every year. Yes, this Dodger lefthander is somebody that baseball fans need to be paying attention to every time he takes the mound. In fact, he is the only active player in the top 15 for ERA+. (The next active guy is Chris Sale at 139.) You’re watching history, folks.

3. Pedro Martinez

Pedro Martinez


While he may not have the strongest grip on the English language (and they still put him on TV), this man knew how to grip a baseball. Posting a career 154 ERA+ over his entire 18 year career, Pedro made his presence very obviously known as one of the most intimidating at bats in the history of the game. During the meat of his career in Boston, he cemented himself into baseball lore – and deservedly so. He was 50% percent better than everyone for the length of his career, even factoring in his rocky two year exit from the game.

Will it change the game FOREVER?

Well, the thing is, it's hard to get people to change their ways. Especially when their current way is simple and the way you want them to change is hard. So will you start seeing ERA+ as one of the main stats on a pitcher's bio now? Probably not.Too much math involved and people are used to seeing good ol' fashion ERA, anyway. However, behind the scenes, you better believe that the front offices around the MLB know all about ERA+ and how they can use it to their advantages.

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