Formula for xERA

[tag0519]

Does anyone know the formula for xERA? I recall seeing it a while back. I recall there being some seasonal-specific constants and a normalizing factor in there.

Thanks in advance,
-Tom

[Ubiquitous]

((.575 * H/9 ) + (.94 * HR/9 ) + (.28 * BB/9 ) - (.01 * K/9 ) - Normalizing Factor). The normalizing factor is based on the league (typically in the .270 and .285 range).

That is the old version. Apparently they have come out with a new version that doesn't require a normalizing factor anymore but it appears they've turned it into a black box metric.

[Jackaroo Dave]

((.575 * H/9 ) + (.94 * HR/9 ) + (.28 * BB/9 ) - (.01 * K/9 ) - Normalizing Factor). The normalizing factor is based on the league (typically in the .270 and .285 range).

That is the old version. Apparently they have come out with a new version that doesn't require a normalizing factor anymore but it appears they've turned it into a black box metric.

If you google it with "expert," you will get a 2010 version that is quite different from the above. It's not from the xera website, so I don't know if it's canonical or not.

Can I ask you guys? What do you make of what amounts to a string of magic numbers? I feel that it's probably expectation of past events--shame on me for not digging in and finding out. And, I can't see it generating much insight about how games are won or lost. Obviously you see some value in it that eludes me. Spare a clue for the old guy.

[Jackaroo Dave]

((.575 * H/9 ) + (.94 * HR/9 ) + (.28 * BB/9 ) - (.01 * K/9 ) - Normalizing Factor). The normalizing factor is based on the league (typically in the .270 and .285 range).

That is the old version. Apparently they have come out with a new version that doesn't require a normalizing factor anymore but it appears they've turned it into a black box metric.

If you google it with "expert," you will get a 2010 version that is quite different from the above. It's not from the xera website, so I don't know if it's canonical or not.

Can I ask you guys? What do you make of what amounts to a string of magic numbers? I feel that it's probably expectation of past events--shame on me for not digging in and finding out. And, I can't see it generating much insight about how games are won or lost. Obviously you see some value in it that eludes me. Spare a clue for the old guy.

[tag0519]

Thanks Ubiquitous and Jackaroo.

Jackaroo - i think those numbers are based on some sort of league average.

Thought you guys would like to know. I dug up this formula for the new xera....

xER = xER%*(FB/10) + (1-xS%)*[(0.3*BIP-(FB/10))+BB]
where
xER% = 0.96 - 0.0284*G/F
xS% = 64.5 + (K/9*1.2) - (BB/9*(BB/9+1))/20 + (0.0012*GB%^2 - 0.001*GB% - 2.4)


Thanks again,
Tom