Advanced Metrics and How to Use Them: Fielding

Tampa Bay’s center fielder Kevin Kiermaier led all of baseball in both UZR/150 and DRS. Mandatory Credit: Jennifer Buchanan-USA TODAY Sports

Welcome to the third installment of Advanced Metrics and How to Use Them. Having already looked into batting and pitching, we’re going to tackle our biggest challenge yet: fielding and defense. Let’s get into it!

Fielding

Figuring out how well fielders do their job is far and away the most challenging aspect of determining value. While we’ve gotten pretty good at calculating the value that pitchers and batters provide, we’re still learning how best to measure defense. Hopefully, Statcast and the data that it provides will revolutionize how we handle these things, but that’s at least a few years out. In the meantime, let’s look at what we do know.

The Bad: Errors & Fielding Percentage

Long held as the gold standard and only way to measure fielding, errors and fielding percentage can’t be all bad, right? We all grew up using errors, why would we want to change now?

The Problem: The MLB rules spend about 1500 words trying to define what an error is, exactly, and at the end of it all, the whole thing is pretty fuzzy. There are some clearly defined errors, like throwing the ball three feet above the first baseman’s head, and letting a dribbler roll between your legs, but the long and the short of it is that it frequently comes down to an official scorer deciding whether or not a fielder should have made the play with “ordinary effort.” It sounds great, but what is “ordinary effort”?

Let’s take an example. I remember a play in which Peter Bourjos had a ball bounce in and out of his glove in center field. Oops. Clearly an error, right? Well, I also remember that he overran a ball that I’m not sure anyone else would have gotten to at all. An “average fielder” making “ordinary effort”? No way. What about the other way? What if Holliday with his lightning speed fails to corral a ball in left? Is that an error? How do you tell whether an “average fielder” making an “ordinary effort” gets there?

In short, what is and isn’t an error becomes very subjective very quickly, and frequently the stone statue with steady hands gets the better end of the valuation.

How to Use Them: Errors and fielding percentage aren’t useless, by any means, and can do a pretty good job telling us who has good hands and a steady arm, but they fail to account for range effectively. So if you’re looking to judge overall defensive prowess, this isn’t the best tool, but finding the outliers at either end of the spectrum can tell you who’s especially jittery or especially sure.

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The Good: Ultimate Zone Rating/150 & Defensive Runs Saved

Ultimate Zone Rating/150 and Defensive Runs Saved? Those sound pretty convoluted and hard to understand. What do they do and why should we all care?

UZR/150: UZR stands for Ultimate Zone Rating, an attempt to quantify a player’s defense by dividing the field into “zones” and then judging how many plays are made collectively in each zone. By comparing each player’s performance against that of the rest of the league, you can start to get an idea of how well a player is performing. There are a number of aspects to it — including range, arm, errors, and park factors – and after a complex calculation, the formula spits out a number. Average is 0, and every positive number is above average while every negative number is below average. When you scale it to be based on 150 games played, you get UZR/150, which assures that we’re comparing the same thing since UZR is a counting stat that accumulates slowly.

So what makes this worth while? Although it still is based on human charting and is thus subject to human error, dividing the fields into these zones still gives us a much more accurate picture of how well a player performs. We have a way to quantify one player against another, even if it isn’t perfectly accurate.

DRS:  Defensive Runs Saved is a statistic calculated by The Fielding Bible that attempts to measure the number of runs a player saves on defense. Just like UZR, it sits on a scale where 0 is average. It tries to factor in all of the same things that UZR does, but sometimes comes up with different results due to a different formula. The reason I use both of them is because they collectively provide a better picture than either would individually.

How to Use Them: Defense is really tricky to calculate, but UZR/150 and DRS make an admirable attempt to do just that. Look at them when you want to examine a player’s defense, but also remember that they’re best used when understood when used in a broad stroke. For instance, Jason Heyward is a really good right fielder. Did he really save exactly 22 runs and not 21 or 23? Maybe. But let’s settle for saying that he’s a really good right fielder. Additionally, it takes quite a while (probably multiple seasons) for defensive data to stabilize properly, which means that no one should be drawing conclusions from 50 or 100 innings worth of data. Remember those two things, and you’ll be fine.

An Additional Note on Catchers

Catchers: The guy behind the plate is a different beast entirely. While DRS is calculated for catchers, everyone acknowledges that it isn’t capturing the whole picture. Recently, some studies on pitch framing have indicated that good catchers may be saving an additional 10-20 runs (that correlates to 1-2 wins) a season with that skill alone. Additionally, most who study baseball understand that “controlling a game” and “working with a pitching staff” are real things, but no one has come up with a good, consistent way to measure those effects yet. Long story short, if you want to judge a right fielder or a shortstop, you have some good numbers to work with here, but if you want to try to understand catching, you’ll have to dig deeper.

As always, if you’re interested in learning more, I’d encourage you to check out Fangraphs. In addition to wonderful articles and lots of player data, they have a phenomenal glossary that offers great resources for further learning and exploration.

Next: Byung-Ho No?

So there’s a very quick look into the challenging world of evaluating defense with advanced metrics. Next week, we’ll address the big one: WAR.