Introducing Statistics: Ultimate Zone Rating
Fielding statistics in general have been the black hole of sabermetrics. Branch Rickey, recognizing the importance of fielding, still said “[t]here is nothing on earth anybody can do with fielding.” Fielding statistics stayed remarkably static from the earliest days of baseball, when fielding percentage was invented. Fielding percentage simply takes errors and divides by total chances. The fewer errors, the higher the percentage. Unfortunately for the stat, players in general make less errors each year due to advances in baseball gloves and the quality of the field of play. This has led to a search for stats that better quantify fielding. I am going to focus today on one such attempt, ultimate zone rating (UZR).
UZR was developed originally by Mitchel Lichtman. It attempted to advance beyond range factor, a fielding stat developed by Bill James calculated as (putouts + assists)/games played. UZR attempted to correct for several problems with range factor, first by using innings as a measure instead of games and second by controlling for the effects of pitching/luck. UZR attempts to measure how well a fielder can turn a batted ball into an out. In particular it measures this as the number of runs saved.
UZR divides the fields up into a variety of zones, gives individual fielders responsibility for those zones, and calculates how often a hit into each zone is turned into an out. By comparing the probability of an out being made, on average, and the number of outs that a fielder actually converts, you get the number of runs that a fielder saves above average. From this starting point, FanGraphs adds double play runs, outfield arm runs, and error runs, three categories left out of the original calculation. It also reports a rate stat, UZR/150, which number of runs saved per 150 defensive games. This is intended to correct for part timers and the fact that very few players play 162 games in a year.
UZR has one large minus: It is almost completely inaccessible to any but the most devoted sabermetric fan. Casual fans can understand it and look it up on sites that list it, but they don’t have access to the data necessary to calculate it easily. This is part of the explanation for the persistence of fielding percentage; it is easy math. It is also ahistorical; the data does not exist far enough back for meaningful comparisons to be made about players from different decades, let alone eras. However, it has the advantage of incorporating range into fielding statistics in a way that fielding percentage never did. It also allows for a more complex understanding of range than range factor. UZR, then, is a useful current stat, but it is a serious challenge to comprehend or calculate for the casual fan.