Posted by Sean Forman on March 25, 2010
Right now on the site you'll see different ERA+ numbers from what you are used to seeing on the site. The old formula was 100*(lgERA/playerERA). The numbers you see now are 100*(2- playerERA/lgERA). This changes the numbers somewhat and bunches the top end a bit more, but doesn't change the ranking of players. The two lists of league leaders are the exact same.
I had not intended to roll out this change just yet as I was mulling a change in name to show that we are presenting a different formula. I was testing the change and ended up rolling it out to the site unintentionally. I apologize, and I am embarassed by the confusion this has caused.
The reasons for making a change are a bit esoteric, but I find them compelling. The old formula is a power equation. The independent variable is in the denominator, so you get a 10% change in the player's ERA showing up as any of a variety of percentage changes in ERA+. It depends where on the curve you are. For example, if you have a league ERA of 4.50 and one pitcher at 3.50 and one at 3.00 and one at 2.50, you get ERA+'s of 129 and 150 and 180. The changes aren't linear.
With the new formula, the equation is linear, so if the league ERA is 4.50 and you have one pitcher at 3.50, one at 3.00 and one at 2.50 you get ERA+'s of 122, 133 and one at 144 (one is 22% better than the league, one is 33%, and one is 44% better). It seems to me the numbers make a little more sense this way.
That said, I've rolled this out in about the worst possible way, so for now, I'm going to take the day to figure out what to do and then implement it tomorrow. I apologize again for the confusion this caused.
UPDATE: As I said above, it was not my intent to roll this out at this time. I still believe this to be a good idea, but it needs to be done in a MUCH more organized manner, so I'll be rolling back to the old stat tonight or tomorrow and then taking a more measured approach going forward. I apologize again for the confusion.