Photo- Keith Allison/Getty Images

Statcast recently released their newest toy: Infield Outs Above Average (IOAA), and it has not disappointed. Designed by sabermetrics legend Tom Tango and the MLB Advanced Media (MLBAM) staff, IOAA is a major development for defensive metrics, finally eliminating the biggest hurdle for quantifying infield defense – positioning. For an in-depth discussion on IOAA from MLB’s Mike Petriello and Tom Tango himself, check out the most recent Statcast Podcast. For an easy-to-read breakdown, check out Mike Petriello’s article on MLB.com. For a thorough breakdown of both IOAA and the history defensive metrics, Tango’s 21-page thesis on his website is the best resource available (seriously, take the time to read this – it will make you smarter).

Previously, defensive metrics have usually eliminated shifted plays entirely since they weren’t able to accurately account for positioning. The problem with this is that teams are shifting more than ever before. Per Statcast, we actually saw the 2019 Dodgers become the first team to shift 50% of the time, so it’s obviously difficult to accurately measure defensive value if half of our sample is eliminated.

Note: Fielding Bible’s updated DRS metric has also made major strides in this area, using their new PART system to eliminate positioning from their formula. This is another fantastic resource for infield defense.

So, how does this thing work?

For a much more thorough description, use the resources above. If you don’t care to do that and would instead prefer to see a quick/subpar description from yours truly, IOAA uses three “action points” to estimate the defender’s probability of success. The three action points are:

The fielder arriving at the ball (a.k.a. the “intercept point”) The fielder retrieving the ball cleanly The race to the bag (on ground balls) – the probability of the ball beating the runner to the base given the distance of the fielder from the base and the speed of the runner

This system is extremely intuitive, so it doesn’t take any special skill set to be able to understand how it works since it’s mostly built around distance and time. Basically, Statcast freezes the play at each action point and asks “what percentage of the time is this type of play converted into an out?” These probabilities are then multiplied together to get the “estimated success rate” of that particular play, and the fielder is credited (or debited) accordingly. So, if Fernando Tatis Jr. converts a play with a 10% estimated success rate into an out, he is credited with 0.9 outs. If he doesn’t make that play, he is debited 0.1 outs. Pretty straightforward, right? This is referred to by Tango as the “distance/time intercept model”. There’s obviously more to it than that and you should absolutely read about the construction of this metric from the folks who designed it, but that’s the basic concept of IOAA.

How do the Padres stack up according to IOAA?

The 2019 Padres were, um… bad. The Padres finished with a combined total of -23 IOAA, which was dead last in baseball. Of course, some fielders were better than others, as shown in the table below. The current data available goes back through the 2017 season, and the Pads were actually pretty good in 2018, but since the Padres did not have Machado, Tatis Jr. or Jurickson Profar prior to last season the table only includes 2019 numbers (Profar had -3 IOAA in Oakland). You’ll immediately notice that a particular middle infielder struggled mightily in 2019, but it’s important to provide some context, so the rest of this article will be dedicated to a closer look at our favorite shortstop.

Here’s the part you were waiting for – Fernando Tatis Jr.

If you’ve paid any attention whatsoever to IOAA thus far, you’ve likely heard that Fernando Tatis Jr. was absolutely atrocious in 2019. He finished with -13 IOAA, which is very bad, especially given that he managed to accumulate this total in only a half-season. Tatis Jr. had an estimated success rate of 88%, meaning an average player would’ve converted roughly 88% of the total opportunities into outs, while Tatis Jr. converted only 83% of opportunities into outs. This was mostly due to errors, as he had 14 throwing errors and only four fielding errors. But as always, no single number is going to tell the whole story, so we’re going to take a deeper look at this.

Directional value

One of the many great features of OAA (and IOAA) is that it breaks down performance by direction. This way, we can tell if a player excelled or struggled with balls that were hit in a particular direction with respect to their starting position. This information can serve a lot of purposes (it could be extremely useful for positioning), but it’s a very effective way to evaluate a player’s defensive strengths and weaknesses in a way that we haven’t been able to in the past using other defensive metrics. This is very important, particularly for Fernando – check out his directional IOAA:

So, Tatis Jr. was pretty much average, except on balls hit directly in front of him. This is where Baseball Savant’s video database comes in handy, especially for a player who struggled with errors. Since it’s easy to search, identify and view those plays, we’ll take a look at some of them in a moment. But we saw him make lots of fantastic plays that would surely create positive value, right? So, let’s take a gander at his numbers when we break it down by degree of difficulty. We can do this by simply separating all of his measured opportunities by estimated success rate.

Value by degree of difficulty

Tatis Jr.’s splits get pretty weird when we divide his opportunities by degree of difficulty. He really struggled with easiest plays (plays that are converted into outs 75-100% of the time), coming in at -9 IOAA on those plays. On the other hand, he was slightly above average on the most difficult plays (0-25% estimated success rate) with +2 IOAA on those opportunities. This would lead us to conclude that range and arm strength aren’t the issues here. You may have heard folks say “he makes the highlight-reel plays but not the easy ones”, and the numbers pretty much agree, at least based on the data we have on him for 2019. Cross-referencing with video is always a good way to confirm what the numbers tells us. So, using the search feature on Baseball Savant I was able to pull video of every single error Tatis Jr. made in 2019 and try to figure out why he has these strange IOAA splits shown in the table above. Here are a couple of examples:

Source: Baseball Savant



Here, Tatis Jr. gets a great read on the ball, gets around it to give himself a good angle, and fields it cleanly. Neil Walker isn’t exactly a burner – his average sprint speed of 25.6 ft/s is good for the 23rd percentile – so Tatis Jr. has plenty of time to gather himself, but he rushes a bit and throws off-balance, ending up with an error. A lot of the errors Tatis Jr. made last year were similar to this. This is the kind of stuff that’s easily fixable for a young player who’s still developing.

Source: Baseball Savant



Here, Kevin Pillar – who is an above-average runner, with an average sprint speed of 28.0 ft/s in 2019 – hits a slow roller to short. Fernando charges and appears to be preparing to make a barehand play, when he seems to change his mind at the last moment and kind of gets caught in between, resulting in a misplay. He doesn’t really need to barehand this ball since he gets to it quickly enough to field it with his glove and make the throw, and the ball is coming in hot enough that it’s going to be tough to barehand this ball cleanly.

Video analysis is completely subjective, of course, but the video seemed to show that he didn’t necessarily struggle with any particular type of play. That is, he rushed some throws, failed to set his feet and made some off-balance throws when he didn’t really have to, and just used sloppy technique at times. This should actually be somewhat encouraging, since it’s a lot easier to improve throwing accuracy than something like range or arm strength. Tatis Jr.’s defense as a whole should be fixable, assuming he is indeed capable of throwing a baseball accurately. Ultimately, we’re looking at a half-season worth of defense from a then-20-year-old kid, and we shouldn’t read too much into it even if his defense was horrible by IOAA and every other defensive metric in 2019. Tatis Jr. is off to a very poor start defensively, but he should have plenty of time to change the narrative, and we all know he has the tools to do exactly that. Oh, and if you were wondering, Luis UrÍas was an average infielder with +1 IOAA.

All stats, videos and information on Infield Outs Above Average are courtesy of Baseball Savant unless otherwise noted.

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