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Thursday, February 25, 2010
Matt on DIPS
Everything he says in the first half, I’m on board with, especially this:
The reality is that everybody who can get hitters to whiff enough to hold a roster spot on a major-league team has similar skills at preventing hits on balls in play.
DIPS is so powerful because the 75% of the time you put the ball in play, you better be darned good at it overall. If you keep getting hammered, you won’t even have a chance at making MLB.
This however:
This is the reason that tRA did so poorly at predicting ERA the following year compared to FIP, despite having all of the same information and batted ball rates mixed in. Since tRA asked the question, “What would the average pitcher’s ERA be, given his strikeout, walk, home run, pop-up, ground-ball, non-HR outfield fly ball, and line-drive rate?” it was given an answer that highly correlated with line drives. There is a negative -0.23 correlated between line drive in a given season and ERA for pitchers who pitched at least 40 innings, but line-drive rate does not carry over to the following season. Thus, any DIPS statistic that relies on line-drive rate will unravel the following season if it tries to predict ERA. That is why when tRA was compared to FIP in predicting the following year’s ERA, it did worse. It uses all the same information, and a bunch of extra information to confuse itself. Basically, tRA is FIP having a nightmare.
I introduced Batted Ball FIP (bbFIP) last week. The equation is pretty simple:
ERA = 11*bigs + 3*smalls + constant
where
bigs = [(BB+LD) - (SO+iFB)] / PA
smalls = (oFB - GB) / PA
constant = whatever you need to align to the league
BB = BB-IBB+HBP
Line Drive is in there. And according to my testing of bbFIP and FIP and SIERA:
1.05 bbFIP
1.05 SIERA
1.11 FIP
And Brian’s results:
Year FIP bbFIP qERA bsrERA SIERA Tango wERA
0 0.743 0.840 0.898 0.924 0.883 0.908 0.694
1 1.076 1.026 1.040 1.037 1.010 1.003 1.180
tRA as best as I understand, is similar to bbFIP in terms of parameters used. So, I don’t think Matt is correct in what he said. It simply doesn’t follow my testing and Brian’s testing. My guess is that there may have been a coding issue, perhaps with the way they calibrated tRA? I dunno. But, it just doesn’t sound right.
I also want to point out that the “Tango” that Brian mentioned, which is simply ERA = 11*(BB-SO)/PA + constant did the best! That is, ignore all aspects of batted ball (INCLUDING HOMERUNS!), and focus only on the 25% of the PA that result in a walk, strikeout or hit batter, and it does as well or better than anything out there.
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