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Wednesday, September 27, 2006
Quick ERAs
I have two versions of component-based ERA.
The first was based on Voros’ DIPS, and I call it FIP:
ERA = (13*HR + 3*BB - 2*SO)/IP + 3.20
You can include HBP and exclude IBB if you like. The 3.20 is a constant and should be altered year-to-year.
The second one came about while talking with Guy at Fanhome, and I call it szERA:
ERA = 5.40 - 12 * (SO - BB)/BFP
Again, decide how you want to handle HBP and IBB. And again, 5.40 should be altered to ensure it all adds up at the league level.
Nate Silver shows his Quick ERA as:
ERA = (2.69+K%*(-3.4)+BB%*3.88+GB%*(-0.66))^2
He correctly notes that run scoring is not linear, which is why he introduced the exponent of 2 (though 1.5 might work out better). However, the GB% in the equation has a different denominator than the K and BB percentages (BIP as opposed to PA). Instead, I would do “GB minus FB” per PA. The run value between these two is around .10 runs, which makes it about one-sixth the run value of a walk minus K. Therefore, my szERA could be modified as:
ERA = x.xx - 12 * (SO - BB)/BFP - 2 * (GB - FB)/BFP
That x.xx would need to be set accordingly. I haven’t tested it, but I’m pretty sure this would work. (Maybe that “2” should be a “4”. Not sure.)