Defending Jacoby Ellsbury
In 2008, Jacoby Ellsbury was rated by Baseball America as the best defensive outfielder in Boston's minor league system. He made good on that prediction in 2008, impressing UZR and posting a 16.8 fielding RAA, 6.9 of which in centerfield.
In 2009, in the season Ellsbury was voted the Defensive Player of the Year by MLB.com, he was ranked by FanGraphs as having the worst defensive year of any center fielder, -18.6 runs. So which is it? Is he the best in the league or the worst in the league? Perhaps the MLB.com award was simply a popularity contest biased by a few web gems; there are a whole lot of Sox fans out there. That's possible, but I'm not quite willing to label Ellsbury the Derek Jeter of center field so quickly. Perhaps UZR isn't properly taking into account the peculiarities of Fenway? This isn't likely, given his rating in 2008 and that of Covelius in 2007.
It's been argued that Ellsbury is Jeter reborn: a poor defensive player who makes up for his deficiencies with flashy plays. The argument is that he makes poor reads off the bat and a poor initial step but makes up for it with his speed and a late diving catch. It's certainly possible, but looking over the video evidence, that's not my read. Some seem ready to dismiss Ellsbury on the basis of his UZR stat alone, but most fans don't seem to be so easily swayed. The fan scouting report over at The Book in 2008 and 2009 lists him as having an above average first step and average instincts (unlike Coco, who amusingly has incredible instincts, but the arm strength of a Girl Scout after a massive stroke).
On the other side, it's been argued that Ellsbury illustrates how meaningless defensive statistics are. No one has ever argued that defensive statistics are as definitive and meaningful as batting or pitching statistics, but to ignore them completely seems unwise. Theo Epstein seems to agree, given his moves to ship Mike Lowell--and his awful 2009 UZR--off to the Rangers while Boston pays his salary. I think the most honest observers have to admit that they just don't know how to reconcile the statistics with the widespread perception that Ellsbury is a good defensive player.
I looked at the pitch f/x data from 2008 and 2009 to get a sense of why Jacoby was being treated so poorly by UZR. Above is a plot of the 6500 plays at Fenway Park in which the x and y coordinates of the hit location was recorded. In red, I've highlighted all plays that involved the center fielder (each of these plays included the CF actually fielding the ball, either before or after it hit the turf. If the CF dove and missed, that isn't recorded by gameday). In each of the two years, we can look at the distribution of hits and outs that were fielded by all centerfielders playing in Fenway.
It's pretty hard to look to interpret these raw data, so I used a kernel density estimator to estimate the likelihood of each hit being turned into an out by either Ellsbury or the average CF playing in Fenway.
The blue circles in the scatter plots show hit locations from 2008 and 2009 that Ellsbury would have been less likely to have turned into outs; red circles show the locations Ellsbury would be more likely to turn into outs. Many of the circles are nearly white because there is no difference between Ellsbury and the average CF.
Gameday recorded a small minority of the actual hit locations, so it would be difficult to say anything conclusively even if there were strong patterns in the data (it's also unclear to me how many at-bats had zones recorded in the data that is used to calculate UZR. Retrosheet is missing hit locations for a vast majority of 2009 in Fenway). But it looks like Ellsbury didn't differ significantly from the average CF in 2008. In 2009, it looks like he may have been weaker at coming in on balls dropping in front of him, but there's no way to tell if that's pure noise. But what we absolutely don't see is the gaping hole you would expect to see in the range of the games worst centerfielder. Ellsbury seems to miss the plays that any CF would have missed, and seems to make the plays that any CF would have made. If I were a betting man, I would guess that Ellsbury's UZR will be league average next year.