Those of you who comment over at RAB might know that I have been slowly backing away from UZR for the last few months as I have learned more about the metric. I still use the metric, but it is with significantly more caution than in the past. My primary objection to the way it is used matches the concerns of Kris Liakos at Walkoff Walk:
With a typical outfielder getting at least twice as many plate appearances as he does defensive chances, when reading advanced stats we’ve all learned to trust a single season’s offensive numbers to paint an accurate portrait of a player, but to factor 3 years of defensive numbers. It makes sense, but it doesn’t solve the perception problem of simply looking at a guy’s UZR numbers and trying to figure what kind of fielder he is right now. Common sense tells you that you can’t simply average the numbers since each year will have a different number of defensive chances, and while the reality of a player that has posted a -14.2, +10, -6.9, +12.1 is that he’s got average range and average arm, it doesn’t look that way on the page.
So to solve the perception problem and stop dummies like me from misunderstanding/misinterpreting the meaning of UZR, I have a humble proposal. Do away with the year-to-year UZR rating of a player, and replace it with a single career number. Beginning in a player’s second year the problem of small sample size will start to dissipate and on a single look you’ll be able to make a quick judgement on just how much, or how little, he’s able to do in the field. To account for diminishing skills with age or trouble playing in a new park, each player’s career UZR can have a little up or down arrow like the Beckett Price Guides of old representing whether his number has gone up or down in his last 400 defensive chances.
To sum up, the problem with the way many use UZR is that they cite single season UZR’s and base other statistics, such as WAR, off of that number. However, the sample is too small to know whether the single season UZR represents an actual fluctuation in perfomance or a simple sampling error. As such, a single season UZR is, at best, a rough estimate, yet is treated as a much stronger indicator of defensive performance by fans and bloggers alike.
Now, Kris’s solution is that we simply replace UZR with career UZR in our analysis, with an up or down arrow representing recent trends to provide for some more immediate context. However, I think that this system would result in a lack of specificity to the extent that the statistic could not be used to measure current value in any real way. Instead, I propose a system whereby a 450 game average is computed each year, with more weight given to more recent seasons. To illustrate, a 2009 UZR would include the player’s last 450 games, presumably including 2009, 2008, and 2007, with 2009 given more weight in the computation than 2008, 2008 more than 2007, and so on. Essentially, each “single season UZR” is in actuality a composite of the player’s last 450 defensive games, which is a healthy enough sample from which to draw conclusions, yet provides specificity and context through the use of the most relevant recent sample and by providing greater weight to the most recent season. Single season UZR could still be utilized, but as a secondary measure used with careful reservations in the context of the 3 year number.
I would love some feedback on this idea. Chime in below.
Graphic Credit: Beyond the Boxscore.com