I think most Astros' fans who watch Michael Bourn play center field think he is a superlative defensive player. Based on watching most of his games, I agree. But a funny thing happened on the way to Bourn's defensive rating. His Ultimate Zone Rating (UZR) for 2009 is -1.6. This is worse than Bourn's UZR in 2008, which was +2. Similarly the Fielding Bible +/- system indicates that Bourn's range has been sub par this year, but that his overall defense has saved 3 runs this year because of his throwing arm.
I think these two defensive metrics are the most sophisticated defensive measures; generally they are the best we have. However, they are just measures, and sometimes they may not be reliable indicators of a players' defense. As the creator of UZR has said, UZR doesn't tell you if a player is a good or bad defensive player, but only whether the player has a good or bad UZR.
When I turned to Hardball Times' fielding stats, something jumped out at me: Michael Bourn's Out of Zone (OOZ) plays are incredible. The baseball field is divided into numerous zones, and defensive metrics assign certain zones to specific positions. Bourn's OOZ refers to the occasions when he converts an out on a play outside of the normal zone for a CFer.
Currently, Bourn leads all major league players at all positions in OOZ. Bourn has converted 81 OOZ plays. The closest number of OOZ in the majors is Franklin Gutierrez with 77. In the NL, the closest OOZ is posted by 3d baseman Ryan Zimmerman with 63. The closest NL CFer is Matt Kemp with 60.
Not only that, but Bourn is on an peerless path for OOZ plays, if his current trend continues. At his current pace, Bourn would convert 128 out of zone play this year.. Since 2004, the highest number of OOZ plays was 111 (Beltran in 2008) and the lowest number of ML leading OOZ plays was 93 (Pujols in 2006 and Tejada in 2004). Clearly, Bourn is using his speed to make plays all over the outfield, even if it's not confined to the normal CF locations.It's also worth mentioning that Bourn's Revised Zone Rating (RZR) is above average for a CFer. RZR is less sophisticated than UZR. But one decision made by Hardball Times was to separate the OOZ plays from RZR (leaving only in zone plays in the RZR score). That decision was based on a reasonable position that no consensus exists on how to compare OOZ plays to in-zone plays. At the time of the decision, HT pointed out that OOZ probably should be weighted to reflect a higher value than in-zone plays. Another issue which arises if OOZ is incorporated into RZR is the interaction with adjacent fielders.
I don't know how Bourn's OOZ plays are weighted in the UZR methodology. UZR does use an allocation process to indicate how many plays should be made by each fielding position for zones in which fielding plays overlap (e.g., zones in which both the 3d baseman and shortstop make plays, or both the CFer and RFer make plays). This raises some additional questions in my mind about factors which may be negatively affecting Bourn's UZR rating. Is it possible that an adjacent fielder with poor range, namely Carlos Lee in LF, is causing Bourn's UZR to be lower than we expect? Perhaps Bourn is ranging far and wide in CF because he knows Carlos Lee can't get to many of the plays falling between CF and LF. To some extent, the same situation may occur in the seam between RF and CF, which, if true, would indicate that Pence can attain a higher RZR rating because he doesn't have to worry about plays closer to CF.
In addition, Bourn has to cover one of the most expansive centerfields around in his home park. UZR incorporates park adjustments, but these adjustments, by necessity, are relatively coarse. Perhaps the expanse of Minute Maid Park and Tal's Hill has an effect too.