
UZR or Ultimate Zone Rating is becoing a standard defensive metric, but I worry a little that it's popularity stems mostly from being readily available at FanGraphs.com. So I was excited a couple of months ago to stumble across and interview with the developer of UZR, Mitchel Lichtman at BaseballDailyDigest.com, conducted by Joel Hamrahi. I can't find it on that site anymore, but what follows is a synopsis of how it works.
UZR starts with a map of the field that divides it into 22 slices and then divides those up into distances of 30-35 feet. (Just so you can picture it, I threw together the dreadful little drawing on the left.) For each of those spots it has been computed from historical data what percentage of the time a ball is turned into outs.
In addition, each of those probabilities are broken down into more granular probabilities based on further conditions. Those other conditions are important, so Lichtman specified them. They are:
- Type, which I think he means as type of hit, but the values are hard, medium, and soft
- handedness of the batter, because it influences positioning and he claims it influences the speed, which I don't quite understand
- game situation, meaning the baserunners and outs, because is also influences positioning
- ground ball/fly ball ratio of the pitcher, which again he says influences the speed.
(The most notable exception in my mind? There is nothing on the type of hit (line drive, fly ball, etc) or on the speed at which the ball got to that zone, which I would think would be fairly important in determining how likely it was to be fielded.)
So, basically you have a huge table that has location and the rest of these conditions as columns, and for every possible combination of those, it has the percentage of time a ball is turned into an out by each fielder. Based on those percentages, a fielder gets or loses varying amounts of credit for their performance. For instance, if Jermaine Dye catches a ball that 90% of right fielders catch, he gets credit for 10% of an out. If he misses a ball that 60% of right fielders catch, he loses 60% of an out.
Then UZR turns those plays into runs using a very high level metric. It counts an out as .28 runs, an infield hit as .5 runs and an outfield hit as .6 runs. Since every ball is one or the other, I'm assuming that a play by an infielder credits or subrtracts .78 runs, and an outfielders play counts as .88 runs. So I think that if we go back to our outfielder who got credit for 10% of a catch, he gains .088 runs for that catch.
Lichtman goes into more detail, and I think a few of them are important. First, he rightly points out that in his system "fielder positioning is inherent in the results." He also only tracks flyballs to outfielders and groundballs to infielders, so an outfielder who misplays ground balls isn't penalized. He also adjusts the final numbers based on the outfielders throws. He adjusts for park factors. And finally, an outfielder is never penalized if a gap hit is caught by another outfielder, but is if it is a hit.
Anoother point that I don't think that Lichtman doesn't go into is that certain parks might penalize the fielders who play them unfairly due to their shorter dimensions. For instance, I find it a little puzzling that Michael Cuddyer got hammered with a -25 UZR last year. Sure, his range isn't great, but it's not brutal either. Plus, he offsets some of that by often holding runners to singles even if they have hits off the short right field wall.