ERA+

[savafan]

Just curious, as I saw this in another thread, but thought it'd be easier to reference in a thread of its own. What is this stat, what does it mean, what does it tell about a pitcher and how is it calculated?

[RedsManRick]

Just curious, as I saw this in another thread, but thought it'd be easier to reference in a thread of its own. What is this stat, what does it mean, what does it tell about a pitcher and how is it calculated?

The short answer is that when you see a + after the stat, it means indexed to league average. Stats with the + are ususally referred to as "Adjusted ____". In a standard index, 100 is average. Numbers above and below that are % better/worse relative to average. It is adjusted for park factors.

I think Baseball Reference recently changed it's ERA+ calculation at the request of many in the sabermetric blogosphere, but it currently displays the calculation as:

100 * (lgERA / ERA)

So if league average ERA is 4.00 and the pitcher's ERA is 3.00, his ERA+ would be 133.

However, I think that they may have changed it recently so that it would be more linear -- by making it the difference from league average that matters rather than the ratio. With the above formula, a 2.00 ERA would be 200, compared to 133 for a 3.00. It's not terribly intuitive since 2 runs below average is "twice" as good as good as 1 run below average.

In any event, the concept is the same. The advantage of using adjusted ERA is that you can compare players from any era by describing how good they were compared to their peers. So you can say that while Bob Gibson & Pedro Martinez have very similar career ERA's (2.91 & 2.93), Pedro was much better considering his era, with an ERA+ of 154 to Gibson's 128.

It's also a quick way to check against park effects. Looking at ERA+ and OPS+, we can see that the Reds of the last decade weren't exactly as horrible on the mound or as good at the plate as we thought of them.

[Redhook]

Thanks for the explanation Rick. I didn't know what it was either. Pretty interesting to see the comp between Pedro and Gibson.