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Thursday, September 02, 2010
The two uncertainties of UZR
Pat says:
UZR liked him for 31 runs above replacement. ... Still, UZR wasn’t alone; DRS said he was even better, at 32 runs saved, and Total Zone liked him for 27 runs. It was truly an incredible year for Guti.... Finally, the glove hasn’t played like it did last year. UZR still thinks he’s been good for 7.5 runs, while DRS is even more bullish with 16 runs…
It’s VERY POSSIBLE that Guti’s glove DID play exactly last year as it is this year. That’s because UZR has an uncertainty in classifying the degree of difficulty of a play.
Then, there’s simply random variation. For example, Pujols can be a true .440 wOBA hitter, and he swings and approached each PA exactly the same, and in the first 600 PA he has a .420 wOBA and in his next 600 PA he has a .460 wOBA, and this does NOT mean that Pujols played better. He played the same, and good/bad luck explains the difference.
I’m not saying that this is what happened in Guti case, or any player’s case. But, it’s important to understand that seeing a UZR number is not like seeing a wOBA number. wOBA has one uncertainty (random variation), while UZR has a second uncertainty (classification of batted balls).
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You also hear about how a park “played” like a hitter’s park, even though it is a pitcher’s park. If players at PETCO for example generated more runs there than away, that doesn’t mean PETCO is now a hitter’s park. It simply means that random variation reared its head (you flipped 10 straight heads). It happens.