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Monday, November 23, 2009
Is UZR park adjusted
Eric makes his case:
If UZR had no park error, this estimate of staff BABIP skill would not correlate with our very reliably calculated Park Factors. But it does so, enormously (r = .47, p = 10^-15). In fact, the best predictor of what UZR thinks is staff BABIP skill is .751 * Park Factor. Which is an awful lot.
I dunno… my head was spinning quite a bit there. I think you would jsut need to correlate UZR to BPro’s park factor for BIP, much like I have it here. In that chart, we see that Coors and Fenway were fielding-unfriendly and Dodger and Yankee Stadiums were fielding-friendly. So, when you run your correlation, if UZR has properly handled the park effect, then the correlation should be close to zero. Eric howver is reporting a high correlation, but ... to something. I don’t like the way he says that if you subtract this from that, you are left with the other thing. Luck is always part of the equation too.
Now, the first thing that jumps out at you is that there’s no way the 2005-6 New York Yankees were both the worst fielding and best BABIP-pitching team in recent memory. They were certainly bad at the former and good at the latter, but the size of the numbers suggests that their UZR for those years was low, maybe way too low, and thus the data is giving their pitchers undeserved credit and Derek Jeter their fielders too much blame.
Equally suspicious are the ‘06-’07 Royals, who are the opposite. The ‘03 A’s, another crazy good-fielding, bad pitching team, are also suspect.
In fact, if UZR were doing a perfect job of separating fielding from BABIP skill (which is precisely what it is attempting to do), these two tables would not correlate at all. In fact, they have a mild inverse correlation (-.18); you can predict the numbers in the second table to a mild but very significant degree by multiplying the first table by .16 and flipping the sign.
I think at the least he’s given us enough to consider in order for us (or MGL) to show that bias does not exist. If it shows that we have an inverse correlation, then we can be pretty sure that the level of adjustment is not enough.