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Defensive Statistics Glossary

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MLB: Cleveland Indians at Houston Astros Troy Taormina-USA TODAY Sports

To close my series on defense, I’d like to put forth a glossary of statistics used to measure fielding. Much like HH did recently for pitching, I’ll provide a brief definition of traditional and advanced defensive stats that are popular. I’ll also share some limitations of the various statistics and how they can be interpreted. Please note: This is not an exhaustive glossary of terms. I am only presenting a selection of defensive stats, mostly those that are trending toward mainstream. Hopefully, many of the terms look familiar to you—especially if you’ve followed my series reviewing the Astros defense in 2018. But if you’re looking for a comprehensive list of defensive terms, you’ll want to visit FanGraphs, Baseball Prospectus, or MLB.com for a more complete glossary.

Of course, I also want to know if there are any other metrics you use—maybe a combination of defensive stats, or a particular weighting formula for various elements of fielding—to measure a player’s or team’s defensive ability. So please share any additional measurement methodologies you have in mind in the comments.

Traditional Stats

Errors (E) – given to a fielder when, in the judgment of the official scorer, he fails to convert an out on a play an average fielder should have made. Fielders can also be given errors if they make a poor play that allows one or more runners to advance on the bases. A batter does not necessarily need to reach base for a fielder to be given an error. If he drops a foul ball that extends an at-bat, that fielder can also be assessed an error.

Putouts (PO) – raw number of outs recorded by the player

Assists (A) – awarded to a fielder who touches the ball before a putout is recorded by another fielder

Total Chances (TC) – the total number of opportunities a fielder had to record an out

Formula: TC = PO + A + E

Fielding Percentage (Fld %) - percentage of time a fielder makes the play, when giving an opportunity

Formula: Fielding % = (PO + A)/(PO + A + E)

Shifts – defensive realignment of fielders in which they “shift” away from their traditional positions. Infield and outfield shifts are tracked separately, but essentially a shift occurs when three or more infielders or outfielders are on one side of second base. There are other scenarios classified as shifts (such as a fourth outfielder), which are explained here.

MLB: Kansas City Royals at New York Yankees Adam Hunger-USA TODAY Sports

Advanced Stats

Defensive Runs Saved (DRS) – used to rate players as runs above or below average on defense—combines the amount of runs a player saved via stolen base, bunts, double plays, outfield arm, homerun saving catches, and range

DRS/150 – used to rate players as runs above or below average on defense, per 150 defensive games (used to standardize evaluations for players who play one inning versus those who play a full game)

Ultimate Zone Rating (UZR) – fairly similar to DRS, but offers a perspective on a player’s defense relative to league average at his position—takes into account the amount of runs a player saved via his errors, range, outfield arm, and double-play ability.

UZR/150 – offers a perspective on a player’s defense relative to league average at his position, per 150 defensive games (used to standardize evaluations for players who play one inning versus those who play a full game)

Fielding Runs Above Average (FRAA) – uses plays made, expected number of plays per position, batter handedness, park factors, and base-out states to measure individual defense

Range Factor (RF) - determined by dividing the sum of a fielder’s putouts and assists by his total number of defensive games played

MLB: Cleveland Indians at Houston Astros Troy Taormina-USA TODAY Sports

Defensive Efficiency Ratio (DER) – a team statistic used to represent the percentage of balls in play converted into outs. Can be approximated with (1 - BABIP), if all you have is BABIP.

Formula (according to MLB.com): DER = 1 - ((H + ROE - HR) / (PA - BB - SO - HBP - HR))

Inside Edge Fielding – a set of categories that represent how often a player at a given position has made a defensive play of a particular difficulty. Unlike DRS and UZR, Inside Edge Fielding stats do not consider the run value of a play, just the difficulty. The plays are grouped into six classifications: Impossible (0%), Remote (1-10%), Unlikely (10-40%), About Even (40-60%), Likely (60-90%), Almost Certain / Certain (90-100%)

Seattle Mariners v Houston Astros Photo by Bob Levey/Getty Images

Catch Probability – according to MLB.com:

“the likelihood that a batted ball to the outfield will be caught, based on four important pieces of information tracked by Statcast. 1. How far did the fielder have to go? 2. How much time did he have to get there. 3. What direction did he need to go in? 4. Was proximity to the wall a factor?”

Plays are sorted into five categories based on their catch probability. 0-25% = 5 Star play; 26-50% = 4 Star play; 51-75% = 3 Star play; 76-90% = 2 Star play; 91-95% = 1 Star play

Outs Above Average (OAA) – the cumulative effect of all individual Catch Probability plays a fielder has been credited or debited with, making it a range-based metric of fielding skill that accounts for the number of plays made and the difficulty of them. For example, a successful play made with a 75% catch probability receives a +0.25 credit; an unsuccessful attempt on the same play receives a – 0.75 debit. The cumulative score at the end of a season represents OAA.

Arm Strength (ARM) – an estimation of how much value a player adds with his throwing ability, using the maximum velocity of any throw made by a fielder

Pop Time (POP) - number of seconds from the moment the pitch hits the catcher’s mitt to the moment it reaches the fielder’s projected receiving point at the center of the base

MLB: Oakland Athletics at Houston Astros Erik Williams-USA TODAY Sports

Exchange - how quickly the catcher releases the ball, measured in seconds

First Step – time elapsed from contact to the fielder’s first movement toward the ball

Distance Covered – total distance covered by a fielder in his pursuit from the time a batter makes contact to the moment the ball is fielded

Route Efficiency – Distance Covered divided by a straight-line difference between where the player was positioned at contact and where the ball was fielded

Cautions/Limitations

1. Defensive data are more prone to error because there is still a fair amount of reliance on human scores to compile data

2. Most defensive statistics are not useful in small samples. They require at least one season’s worth of data to stabilize, but more often are best served with samples of three years or larger.

3. A few of the defensive metrics (e.g., DRS, UZR) do not account for positional alignment, which can result in misleading interpretations

4. There are different iterations for some of the defensive stats. For instance, Baseball Reference (DefEff) and MLB.com (DER) have different definitions/formulae for the team-wide Defensive Efficiency metric. Although they are similar, they are not identical.

So, what did I miss? Are there any other important defensive fielding statistics that should be added to this glossary? Or what other cautions should be applied when interpreting defensive stats and making inferences from them? Let us know in the comments.