UZR

Ultimate Zone Rating (UZR) is one of the most widely used, publicly available defensive statistics. UZR puts a run value to defense, attempting to quantify how many runs a player saved or gave up through their fielding prowess (or lack thereof). There are a couple different components to UZR, including:

● Outfield Arm Runs (ARM) – The amount of runs above average an outfielder saves with their arm by preventing runners from advancing.
● Double-Play Runs (DPR) – The amount of runs above average an infielder is by turning double-plays.
● Range Runs  (RngR) – Is the player an Ozzie Smith or an Adam Dunn? Do they get to more balls than average or not?
● Error Runs (ErrR) – Does the player commit more or fewer errors compared with a league-average player at their position?

The run values in each of these categories are then compiled into one overall defensive score, UZR. Since UZR is measured in runs, it can be compared easily with a player’s offensive contributions (wRAA or similar statistics). While there are other defensive metrics available, FanGraphs uses UZR in its WAR calculations for non-catchers. You may also see UZR/150 around the site, which is simply UZR scaled to an average number of chances for a season.

Calculation:

For the details on how UZR is calculated — i.e. how we can attach a run value to defensive events — see the FanGraphs UZR Primer. It’s very thorough. The specifics can be a little overwhelming, but at it’s most basic level, it’s a measure of the average amount of damage that batted ball would do and how often it is converted into an out, relative to average at the position. So if the average left fielder makes a player 40% of the time on the ball in question and that batted ball (based on location, speed, etc) is worth 0.8 runs on average, fielding it cleanly earns you 0.48 runs toward your UZR (0.8*0.6). It’s a little more nuanced than that, but that’s the view from 30,000 feet.

Why UZR:

This isn’t the right place to debate UZR versus another similar metric, but you should use a metric like UZR or DRS because it is a better representation of defensive value than something like fielding percentage. Even your eyes aren’t going to do a great job measuring defensive performance because you simply can’t watch and remember enough plays a year to have a good sense of exactly how well a player stacks up against the competition. You might be able to judge a single play better than the metrics (although that’s debatable), but your ability to recall every play and compare them is limited. Run value defensive stats like UZR provide you with the best estimate of defensive value currently available  and allow you to estimate how much a player’s defense has helped his team win.

How To Use UZR:

UZR is as easy to read as it is difficult to calculate. UZR tells you how many runs better or worse that player has been relative to the average player at his position. A +5 UZR at third means the player is five runs better than the average third baseman.

There are some reasons for caution, however. First, UZR is relative to positional average so you want to factor in the fact hat some positions are harder to play than others. For that reason we have the positional adjustment, which we add to UZR to get DEF.

The other thing to remember is that UZR isn’t going to work well in small sample sizes, especially a couple of months or less. Once you get to one and three-year samples, it’s a relatively solid metric but defensive itself is quite variable so you need a good amount of data for the metrics to become particularly useful. There’s plenty more to say about this issue, but that’s for another entry. In general, UZR isn’t perfect because it doesn’t factor in shifts, positioning, and can’t perfectly measure everything it needs to, but it’s still among the best options out there.

Context:

Defensive statistics should not be taken as 100% accurate, just like anything. There are plenty of reasons why they might not be telling you a complete story, and the Overview section goes into a lot of detail about that. As far as interpreting UZR, if you’ve gotten to that point, the scores can be broken down into the following tiers. This is a good shorthand way of evaluating a player’s defensive ability level:

Defensive Ability	UZR
Gold Glove Caliber	+15
Great	+10
Above Average	+5
Average	0
Below Average	-5
Poor	-10
Awful	-15

Things to Remember:

● Beware of sample sizes! If a player only spent 50 innings at a position last season, it’d be a good idea not to draw too many conclusions from their UZR score over that time. Like with any defensive statistic, you should always use three years of UZR data before trying to draw any conclusions on the true talent level of a fielder.

● UZR uses Baseball Info Solutions (BIS) data in calculating its results. It’s important to note that this data is compiled by human scorers, which means that it likely includes some human error. Until StatCast data gets released to the public, we are never going to have wholly accurate defensive data; human error is impossible to avoid when recording fielding locations by hand, no matter how meticulous the scorers. That said, BIS data is still the best, most accurate defensive data available at this time, so just be careful not to overstate claims of a player’s defensive prowess based solely on defensive stats.

● Since UZR is a counting statistic like RBIs or HRs, the more playing time a player accrues, the higher (or lower) their UZR will be. In order to compare players with different amounts of playing time, UZR can be  scaled on a 150 game basis (UZR/150). If you want to compare a player with 90 games played to someone with 140, UZR/150 would be the way to do so.

● UZR is park-adjusted, meaning it adjusts for the fact that fielders have to deal with odd quirks in certain ballparks.

Links for Further Reading:

Intro to UZR – Big League Stew

The Mike Silva Chronicles: UZR – The Book Blog

Similarities and Differences: UZR and Dewan +/-

Background to UZR, Pt. 1 – Baseball Think Factory

Background to UZR, Pt. 2 – Baseball Think Factory