An idiot's guide to using defensive baseball statistics to your advantage

When it comes to measuring the many-splendored things that happen on a baseball field, we're mostly on extremely solid footing these days. If there's an exception, though, then it's probably in the realm of quantifying defensive contributions.

To be sure, advanced fielding metrics like Ultimate Zone Rating (UZR) and Defensive Runs Saved (DRS) make a noble and high-level effort. However, it takes a long time for data samples to become adequate, they don't always agree with each other, and they can be befuddled by heavy shifting, as Andrew Beaton and Michael Salfino of the Wall Street Journal recently reminded us.

Eventually, StatCast's defensive data will make its way to the public sphere on a more widespread basis, and that may change everything. For now, though, the average fan is best served by taking three steps to measuring defense ...

Don't solely rely on fielding percentage

Fielding percentage is of course the most familiar defensive statistic. It's the rate at which a fielder successfully handles a thrown or batted ball. That sounds simple and useful enough, but a lot problems are baked into that simple concept.

First, the scoring of errors is essential to the calculation of fielding percentage, and the scoring of errors is a highly subjective process. One obvious example is when a routine pop-up drops between two incommunicado defenders. That's almost never called an error even though it's plainly a fielding miscue. "Ordinary effort" is the phrase that crops up repeatedly in Rule 9.12. That is, a botched play that requires merely ordinary effort is one that's ripe for an error ruling. It's up to the official scorer to determine whether the play was indeed ordinary. Obviously, that gives a wide berth for subjective interpretation, and that of course leads to some puzzling outcomes.

The other problem with fielding percentage is that it doesn't account for range. After all, you can't be charged with an error on a ball you didn't have the skills to reach in the first place. As such, it's entirely possible to be a significantly better fielder than someone else who has a higher fielding percentage. If you're committing errors at a slightly higher rate, but reaching a lot more batted balls thanks to superior range, then you're helping your team more than the player with the higher fielding percentage.

Yes, fielding percentage provides some reasonably useful information, but it's far too flawed to lean on as your only piece of evidence or even your leading piece of evidence.

As for what's better ...

For individual fielders, use Inside Edge fielding data

The Inside Edge fielding data available at FanGraphs is an excellent and easily understandable blend of scouting and statistics. The Inside Edge staff review every fielding chance for every player (twice) and categorize it based on the difficulty of the play. They're categorized based on how often an average major-league fielder at that same position would make the play in question. Here's how that breaks down ...

  • Impossible (0%)
  • Remote (1-10%)
  • Unlikely (10-40%)
  • About Even (40-60%)
  • Likely (60-90%)
  • Almost Certain/Certain (90-100%)

The first thing to know about these data buckets is that the vast majority of plays are either almost certain/certain or impossible. Specifically, league-wide last season Inside Edge tracked 106,144 plays, and 88,172 of those were almost certain/certain or impossible. In other words, the fielder typically has no play on the batted ball or an easy out 83.1 percent of the time. It's the other 16.9 percent, generally speaking, that separate the best from the middling from the worst.

By way of example, let's take a look at a screenshot of Francisco Lindor's Inside Edge numbers for his career thus far ...

Francisco Lindor SS / Cleveland Indians
Season Innings Impossible (0%) Remote (1-10%) Unlikely (10-40%) Even (40 -60%) Likely (60-90%) Routine (90-100%)
2015 865.1 0.0% (8) 0.0% (14) 66.7% (9) 77.8% (9) 84.6% (13) 96.6% (267)
2016 1,364.2 0.0% (10) 26.9% (26) 46.7% (15) 69.2% (13) 89.3% (28) 98.0% (454)
Total 2,230.0 0.0% (18) 17.5% (40) 54.2% (24) 72.7% (22) 87.8% (41) 97.5% (721)

As you can see -- and as you would expect -- Lindor is adept at making every flavor of makeable play. The numbers in parentheses indicate the number of plays he's made in each bucket, and those implied ratios fall in line with league norms. This speaks to a more specific point -- most plays are routine. Anyhow, these numbers accurately reflect the notion that Lindor is one of the most valuable fielders in all of baseball.

With regard to the aforementioned individual metrics UZR and DRS, Inside Edge numbers have a number of advantages. To wit ...

  • Unlike UZR and DRS, Inside Edge doesn't use run values. With UZR or DRS, a play made with two outs and the bases loaded is worth more than one made with one out and the bases empty. In terms of a backward-looking effort to assign value, the UZR/DRS approach makes some sense. However, if we're interested in knowing who's the best fielder on a sustainable basis, Inside Edge is more illuminating.
  • The fact that Inside Edge is visually derived, as opposed to being pulled from play-by-play data, makes is far less prone to missing the effects of the infield shift. Teams of course are shifting more and more lately, and even though play-by-play metrics attempt to correct for it, having eyeballs on each play is a much better safeguard.
  • It's much easier to understand. A lot of fans understandably have trouble wrapping their heads around the "theoretical runs" that are the currency of UZR and DRS, and that makes them much less user-friendly. The Inside Edge numbers, in contrast, are easy to communicate in terms that will make sense to the garden-variety fan.

None of this is to say it's perfect. Subjectivity is involved, and things like throwing arm and manning the pivot aren't accounted for. But when it comes to evaluating the skills and execution that make up the vast majority of defensive activity, Inside Edge is the best choice for individual assessments.

For team defenses, use Defensive Efficiency Rating

If shifts complicate our efforts to gauge fielding at the the individual level, then they downright undermine them at the team level. Fortunately, a simple and intuitive measure called Defensive Efficiency Rating (DER) is here to help (and has been here to help for a long time). DER is the percentage of balls in play that a defense converts into outs. That's it.

That, of course, is entire point of defense at the team level. Whether by optimal positioning, outstanding range, or some combination of both, you want to turn balls off the bat into outs. DER tells you how well a team is faring at that measure. Unlike the deeply flawed fielding percentage, DER accounts for range and the team-wide ability to make the routine play. As well, it doesn't matter whether a team is a heavy shifter or whether they tend to play straightaway -- DER will capture their success or lack thereof. Typically, you'll find that full-season DER numbers square with reputation. Last season, for instance, the Cubs indeed led all of baseball in DER by a wide margin. That's precisely what you'd expect. You can find DER numbers at Baseball-Reference, where it's referred to as "DefEff."

Baseball Prospectus helpfully gets a bit more detailed with its DER data. On its stat pages, you can look at a team's DER on fly balls, ground balls, liners, and pop-ups. As well, you can find DER adjusted to reflect ballpark tendencies.


So until we get our hands on the full delicious capabilities of Statcast when it comes to fielding, the above approaches will serve you best. To best blend simplicity with quality outputs, lean on Inside Edge data for individual fielders and Defensive Efficiency for entire teams. That's the easy way to evaluate defense in the contemporary era.

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