Thursday, May 20, 2010

An Apology to UZR

Several times over the last year, I've tried to explore the details of Ultimate Zone Rating(UZR), a defensive metric, in part because I thought it was unfairly rating Twins outfielders. It started back in October, but if you click back, make sure you make it down to the second comment. It was by Parker Hageman of, whose research suggested that maybe UZR is biased against smaller outfields.

That's because we thought the zones in UZR didn't take into account the particular ballpark they were in. So a 350 ft fly ball down the right field line was judged similarly in park after park - even if that fly ball was a home run in the Metrodome or Target Field, but not in another field. So, somewhat ironically, the larger, more spacious outfields pumped up UZRs because fielders had more chances to make plays, while the smaller ones penalized UZR.

But we were wrong. I'm sorry UZR, I should not have doubted you.

It is apparent I was wrong because has now added a "Split" option to their statistics, including UZR. Here is Cuddyer's for last year. Instead of having the Metrodome hurt him, it helped him. Cuddyer's UZR/150 was just -6.6 at home but -22.2 on the road. All in all he cost the team just 2.7 runs at home and 7.3 runs on the road.

The same thing happens as you look at his statistics year to year, and you also see the same thing in other ballparks, like Fenway's left field. Jason Bay's 2009 season shows him having a better UZR in Fenway than on the road.

So it appears that somehow, UZR is making the appropriate call for smaller outfields. There are other weaknesses we can pick apart, but all stats have weaknesses. UZR's method for handling somewhat extreme outfields is not one of them. In fact, it seems to be a strength.

But there is something else going on here, too, I think. Somehow, Cuddyer's defense got a lot better in 2009. This offseason I'm 99% sure it said that his UZR/150 was -22.1 in 2009, but now it's -13.6? That's not an insignificant change.

In some ways, that's good news, because I think that's a lot closer to what we're really seen from Cuddyer, and demonstrates his defense hasn't hurt the Twins the way other outfielders, like Jermaine Dye, have hurt their teams. But it's also very, very bad news, because I don't know why it changed. Was there an error before? Is there an error now?

If anyone knows, I'd love to hear it.


Ryan said...

Last month there was a firestorm about UZR owing an apology to Jason Bay

They updated the park effects and improved the formula.

Eric B. B. said...

And the official post regarding the update to UZR:

Parker Hageman said...

Whoa, whoa, whoa...before you go citing me as responsible for creating the "bias against smaller outfields", I was only passing along information on the difference between UZR and Plus/Minus that existed at the time.

Here's one description of it from Tom Tango's website from summer 2009:

"*Park adjustments are handled differently- I believe UZR applies blanket adjustment across all buckets, while Plus/Minus has park factors in form of more precise buckets. A ball hit 395 feet to Vector 190 that stays in the park is only compared to all other balls hit 395 feet to Vector 190 that stay in the park. If it leaves the park, it neither helps nor hurts the fielder. Also, we added the “Manny Adjustment”, which removes fly balls hit unreachably high off a wall. We named the system after the Green Monster’s most notable victim, who went from being by far the worst left fielder in baseball before the adjustment to being only arguably the worst left fielder after the adjustment."

UZR on Fangraphs was updated in April 2010 to account for the quirks of the stadiums:

"Park factors have been improved, especially for “quirky parks and portions of parks,” such as LF and CF at Fenway, LF in Houston, RF in the Metrodome, and the entire OF in Coors Field. Of course, park factors in general are updated every year, as we get more data in each park, and as new parks come into existence and old parks make material (to fielding) changes."

I still standby what I said at the time in October, but UZR has now been smoothed out to reflect the smaller dimensions of the playing surface -- which now makes Cuddyer look serviceable rather than the stoic mass that he is in more spacious fields.

I'll accept an apology in the form of a beer for sullying my good name.

Bill@TDS said...

That adjustment seemed pretty friendly to Justin Morneau as well. I remember him as being around 0 to a couple runs below every year, and now suddenly all his UZRs are above zero (one WAY above), and so far in 2010 he's the top-fielding 1B.

John said...

Sorry Parker. I really was trying to give you credit, not sully you. But I'm happy to buy you that beer.

Parker Hageman said...

@John, no apologies necessary, just thought it merited an explanation.

@BillTDS I don't know if the UZR adjustment influenced the numbers on Morneau that much. Plus/Minus has him at a much better number than he has been in the past. I wonder if getting off of the turf is a factor for him.