Resident statistician Dr. Tom Goyne offers his insights into UZRgate.
Several nights ago, Phillies left fielder Raul Ibanez discussed the fielding statistic Ultimate Zone Rating on the MLB Network. Like a lot of players and managers who are asked about advanced statistics, he felt the statistic was inaccurate and unreliable. When asked about how his UZR improved greatly from 2008 to 2009 (-12.1 versus +8.0), Ibanez stated that the improvement mainly speaks to moving from expansive Safeco Field to the cozier Citizens Bank Park, allowing him to be in better position to get to some balls. As a matter of fact, Ibanez opinions are more consistent than his UZR as he espoused similar theories to the Seattle Times last year. [Link]
It's an interesting theory, and Raul could be partially right. Switching to a new organization may have made for improved positioning instruction during games. After all, Davy Lopes, the savvy base-running/first-base coach, also wears the outfield coach hat. Heck, even if Lopes is half as good at that job as he is at coaxing eight stolen bases out of Ryan Howard last year, it probably leads to pretty good positioning for the outfielders.
However, better positioning doesn't tell the whole story. UZR ratings are not like home run totals, they do not fluctuate based on the dimensions and wind patterns of the ball parks in the way that home run rates do. Once a players zone rating is calculated based on the percentage of balls he fielded that were hit into his zones, the zone rating is then adjusted for ball speed, the handedness of the hitter, the ground ball/fly ball ratio of the pitcher, the base runner and outs situation, and yes, park factors. The rating is then converted into runs based on a run expectancy chart.
While positioning -- and not park factors -- likely account of some of the improvement in Raul's UZR ratings, a large part of the fluctuation probably has to do with the bugaboo of all statistics: sample size.
Sample size is to statistics what location is to real estate and it always, always, always has to be considered. To put things in perspective, while Ibanez has 5,866 career plate appearances which lead to a pretty good understanding of his batting prowess, he only has 1,865 chances as an outfielder (316 in 2008, 224 in 2009). Less than two thousand chances over a 13-season career is a pretty small sample size, and therefore, should be looked at with a skeptical eye since it's quite susceptible to year-to-year variations. So yes, Raul should be somewhat wary of fielding statistics, but not as much as he thinks and certainly not for the reasons he thinks.
Tom Goyne authors Balls, Sticks & Stuff and is a contributor to the Phillies 2010 Annual from Maple Street Press, which hits newsstands March 2. Raul Ibanez is one of his favorite Phillies.