A Note on Defensive Metrics
As per a reader request, below you will find a post about some of the defensive metrics I value, though I will start with an important caveat as it pertains to minor leaguers, first. There is very good defensive data available for evaluation of major leaguers, but in general, prospect evaluators almost have to rely on scouting for the minor leagues. The necessary legwork simply hasn’t been done to provide enough data to evaluate statistics defensively for lower levels.
As far as things I look at when it comes to defense, I think UZR/150, OOZ, RZR, Error Runs and Arm are the places where you can really determine and measure run prevention effectively. You’ll note that I don’t lend much credence to Range Factor, as I just don’t think it’s very useful.
First of all, let’s define how we determine what a zone is. If you look at this graphic, you’ll be able to see the field divided appropriately into nine zones. To be more specific, according to THT, a zone is defined by “The areas on a ballfield in which at least 50% of batted balls are handled for outs. Zones are standardized and defined separately for each position.” You’ll need to consider this when looking at the following fielding metric options below.
As defined in this excellent article by The Hardball Times, RZR, or Revised Zone Rating, is a simple principle that determines the outs made on balls hit into a determined zone for each position. This is, of course, a bit of an oversimplication, but at its core, RZR intends to measure the efficiency of a player in recording an out that he “should” record. Simply put, the statisticians behind RZR determine the amount of balls hit into a fielder’s zone and the amount of outs recorded by all third basemen, and determine the average for each position. It then becomes easy to compare whether a player is above or below average at converting outs on balls in their zone. You can determine how “steady” a fielder is by using this metric, as you can see how reliably he can make the “routine” play. I don’t necessarily consider players with the best RZR to be the best fielders, but I do think you can consider them quantifiably reliable.
Going hand-in-hand with RZR is OOZ (Out of Zone Rating), which is a simple recording of the number of outs made by a fielder outside of his designated zone. This rating is really where the steady ends and the spectacular begins. This is a particularly valuable metric because it determines the ultimate range of a player. The ideal combination would obviously be a reliable RZR (average to slightly above) with a strong OOZ. As an evaluator, I don’t want to compromise reliability, but I’d also really like my fielder to have the range to make the big play from time to time, especially when range on a corner can really determine a single from a double, or even better, an out from a double.
UZR/150 (Ultimate Zone Rating over 150 games) is yet another important measure of defense as it evaluates the positive or negative value of a position player at EACH position over a 150 game (essentially full season) period. The best definition of UZR/150 that I can find is given by Baseball Think Factory here. They say the following: “STATS expanded on ZR by creating sub-zones. You can take the average out-conversion rates by sub-zone and apply this rate to the number of balls in play for each fielder for each sub-zone to establish a baseline. This baseline will show the number of outs an average fielder would have had, had he received the same number of balls in play for each sub-zone that our specific fielder received. This is, essentially, UZR, or Ultimate Zone Rating.” I don’t think it’s really terribly important to grasp all this so much as to say that given the evaluation, it is very easy and reliable to say exactly how many runs above or below zero that they are responsible for allowing or preventing. A negative UZR/150 indicates that a player is essentially costing you considerably on the field, so much so that even with a nice offensive output they still might be a negative defensive player. Simply put, it’s very easy to measure how many runs a player produces but it would be foolish not to put equal weight on the number of runs a player allows defensively.
UZR nicely also accounts for Error Rating and Arm within it. Just a quick note on Error Runs alone, according to the Fangraphs Glossary: “The number of runs above or below average a fielder is, determined by the number of errors he makes as compared to an average fielder at that position given the same distribution of balls in play.” This is a bit simplistic, hence the necessity to include it in UZR (and correspondingly, UZR/150) but its value is in the fact that it allows us to neutralize the balls that each player receives and note how they would do on the same amount of balls, ignoring range and zone. Much like RZR, it can tell us how “reliable” a player is at recording outs.
Lastly, there are surely measures of arm strength, but on a yearly basis you can usually just look at assists to determine how good a fielder was not only at recording outs on tag plays and hits, but also in preventing runners from advancing. What arm strength does separately is allow you to avoid fluky assist seasons and predict who should have that effect each year regardless of luck.
