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Defensive Metrics: Examining How to Evaluate the Game’s Fielders


Cardinals' 2B Tommy Edman; Photo via All-Pro Reels

Judging defense in baseball, especially at higher levels, has always been extremely difficult. Outside of fielding percentage and errors, no counting stat easily determines a player's defensive success. Over time, advanced defensive metrics like UZR (Ultimate Zone Rating) and DRS (Defensive Runs Saved) developed. The emergence of OAA (Outs Above Average) has changed the landscape regarding defensive evaluation, as it is seen as the new best stat for evaluating defenders. However, knowing and understanding all aspects of the stats listed above is essential when evaluating a player's fielding.


Eye Test:

Throughout most of baseball history, the main way to judge a player's defense was simply by the eye test. Now, I am not going to sound like a sabermetric geek and say that the eye test is useless. That assertion is wrong. However, just looking at a player's defense by the eye test is useless, especially at higher levels of baseball. If you watch most MLB players take reps at their position, they will look very good. This is because they are professional athletes. Almost any MLB player can look very good defensively on any given play. This became very obvious to me when I watched an Orioles game in August last season. Orioles 2B Rougned Odor was flying all over the field, making a few outstanding plays. Based on the eye test, one would think Odor is a great defensive 2B. The announcers thought the same thing, as they said something along the lines of “this is why Odor is considered one of the best defenders in baseball at 2B.” This caught me off guard, so I went to baseball savant, and discovered Odor ranked in the 9th percentile in OAA. The eye test did not work this time around. Odor looked like an incredible defender because he is an incredible athlete. However, more is needed to tell the full story of his defensive abilities.


With that being said, it is still important to watch a defender. The eye test gives you some knowledge about how a defender plays moves, which is not disclosed in any stats. While the eye test is not enough to completely evaluate a professional defender, it is still an important tool in doing so.



Errors and Fielding Percentage:

The two most basic stats used for defensive players are errors and fielding percentage. These are extremely basic and are virtually ignored by most MLB organizations nowadays. The main reason these metrics have been deemphasized is that they do not account for the range of a player. For example, a player can have 15 balls hit in his vicinity and only field 10 that were hit right at him. If he cleanly fields all 10 of those hits, he will have a 1.000 fielding percentage and 0 errors but was unable to make a play while moving left to right. However, another player can get to more of the balls in his vicinity. He cleanly fields the ball on 13 out of 14 attempts. That leaves him with 1 error and a .930 fielding percentage. The second player's range clearly makes him the better defender, but errors and fielding percentage indicate the first player is a superior fielder. Both fielding percentage and errors are outdated and don’t take into account a player’s range, making them unfit to accurately value a player’s defense.


UZR and DRS:

UZR (Ultimate Zone Rating) and DRS (Defensive Runs Saved) are two of the prominent advanced stats used for defensive players. UZR attempts to gauge a player's defensive performance by showing how many runs he saves. Similar to fielding percentage, it takes into account errors. However, UZR also considers range, arm, and double-play ability. The formula for UZR is complicated, so instead of trying to put it in my own words, it is easier for MLB.com's glossary to explain it. “UZR uses Baseball Info Solutions data to chart where each ball is hit. Say, for instance, a center fielder sprints to make a nice catch on a fly ball. Then, say data from BIS tells us that similar fly balls get caught 60 percent of the time. That center fielder gains, essentially, 0.4 bonus points for difficulty. If he can't make the play, he loses 0.6 points. At the end of the day, that player's overall score gets adjusted to the league average -- and then that score gets adjusted for how many runs the once-adjusted score is worth.”


DRS is very similar to UZR. They both aim to accomplish the same thing - showing how many runs a defender saves. However, the formulas differ a little bit. UZR tends to have a smaller magnitude and is based more on averages while DRS is more extreme in giving a positive or negative score. However, they will still likely be similar for most players due to the similarity of the stats. One big drawback of DRS is that it does not take into account shifts. For example, a third baseman normally records an out when balls are hit down the third base line. However, if the infield was shifted and the third baseman was playing in shallow right field, no player would be there to field that ball. In this instance, the third baseman’s DRS would be penalized because he didn’t make the play, even though he was on the other side of the infield. This has become a big issue as of late since teams are shifting their infield more than ever. The positioning of fielders is different for every batter, making it tougher to determine whether or not a play should be made. The new shift rules will cause an end to the extreme shifts, but minor shifts will persist, meaning that there will still be shortcomings, albeit less significant, in the calculation of DRS.


DRS and UZR are solid stats and a huge step up from errors, fielding percentage, and the good ole eye test. However, the failure to account for positioning is still a big issue and is a drawback of both UZR and DRS


OAA:

OAA stands for Outs Above Average. It is new but has quickly become the favorite defensive stat of many throughout the game of baseball. Rather than measure a fielder’s effectiveness in runs like DRS, OAA is measured in outs and has different calculations for infielders and outfielders.


First, let's look at the outfield calculation. OAA for outfielders is primarily about one thing: catch probability. This takes into account the distance an outfielder will travel to the ball, the time he has to get there, the direction he travels, and the exit velocity of the ball to give a probability of completing a catch. If an OF makes a catch with an 80% catch probability, he gets plus 0.20 credit towards his OAA. He will get -0.80 credit if he misses the catch. This adds up throughout the year. Good outfielders typically have an OAA of 5-10, but any positive OAA is acceptable.


For infield defense, it is a different story. OAA for infielders takes into account how far a fielder must go to field the ball, how much time he has to get there, and how far he is from the base the runner is heading to. This is much better than DRS/UZR because, due to Statcast positional data, OAA takes into account the positioning of the infielder, which, as mentioned above, is very important in today's game. Overall, OAA takes the positives of UZR/DRS and greatly improves upon them.


 

Defensive metrics have changed drastically over the years and still have a long way to go. It is much tougher to evaluate defense than hitting and pitching, which means it has taken longer to develop reliable defensive metrics. OAA is a huge step forward and is seen as the future of defensive analytics. We are firmly in the “Statcast era” of baseball, where exit velo and spin rate have become fundamental hitting and pitching evaluation metrics. I believe OAA is the next foundational piece of the era, becoming the primary method for evaluating a fielder’s effectiveness.



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