Opportunity Matters for Defensive Value by Ben Clemens December 10, 2019 Picture a beautiful defensive play in your head. For me, it’s Andrelton Simmons charging, slightly to his right. He picks an awkward hop cleanly, transfers it smoothly, and fires a strike to first base to get a speedy runner: Maybe you’re the outfield defense type — you see Victor Robles at a full-out sprint, a seamless transition to a slide, and a line-drive double transmogrified into an out. Either way, beautiful defensive plays are a combination of impossibly fast-twitch muscles and learned grace. Next, picture a team that excels at run prevention. This one is more abstract, so I’ll guide you a little. The best four teams in terms of ERA- last year were all among the top six in strikeout rate. They were all among the top six teams in preventing walks. In fact, out of the top half of the league when it comes to ERA-, only four teams had a below-average strikeout rate, and two of those were almost exactly average. That makes sense — defense can help your pitching out on the margins, but not as much as good pitching helps itself out. But what’s less clear is how exactly this relationship works. Strikeouts keep the ball out of play, and defenders can’t accumulate web gems or defensive stats or whatever your preferred metric for astonishing defense is without the ball. I was curious just how much the pitching staff matters for defenders behind them. At hypothetical extremes, of course, it matters a lot; a pitching staff that struck out or walked every batter they faced wouldn’t need defenders at all. A team that allowed only grounders would sacrifice a lot on the offensive front to have a good defensive infield. But what about actual examples, not silly hypotheticals? Take Simmons. He’s the best shortstop in baseball by general acclaim. Injuries shortened his 2019 season, but even then he was 10 runs above average by UZR and 14 by DRS. That’s a lot of runs saved, of course. It’s also his worst season with the glove. Of course, “runs saved” is an abstract concept. Simmons isn’t using some magical power to make opposing runs simply not count. He’s not surreptitiously adjusting the scoreboard when no one’s looking. He’s just turning batted balls into outs more efficiently than other fielders. How efficiently? That’s the secret sauce of UZR and DRS, and worth reading about, but we’re going high level here, so we’ll ignore it for now. Instead, let’s look at it another way. Simmons had 223 balls hit to his zone in 2019. That was over 873.1 innings in the field, which means he got a chance more or less once every four innings. This roughly matches with the team’s overall rate; Angels shortstops got 0.247 chances per inning overall, while Simmons got 0.255 chances. We already know that Simmons was worth (averaging the two metrics) 12 runs above average on defense in 2019. At 223 balls hit to his zone, that works out to something like a run saved per 20 chances; he saves .0515 runs relative to the average shortstop per ball hit his way. Now, that sounds vanishingly small, and it is. It doesn’t have the same visceral wow factor as watching a defensive play. But just because it isn’t exciting doesn’t mean it doesn’t matter. Simmons is worth about .05 runs above average per chance he gets. Tim Anderson was perhaps the worst defensive shortstop in baseball; he was worth roughly -.05 runs per chance. The difference between the best and worst shortstop defenders is around a tenth of a run per chance. When you take into account that Simmons gets around two chances per game, it almost feels wrong. The difference between the best and worst shortstops in all of baseball is a run every four or five games? It certainly doesn’t feel that way when watching. What do these narrow margins mean? They mean opportunity is king. Want to provide more value on defense? Allow more grounders. Want to allow more grounders? You might have to strike out fewer batters. It’s all connected. Take the Indians, for example. They were among the best pitching staffs in baseball this year. They struck out a lot of batters, and their four-seaming ways led to a high fly-ball rate. Overall, they allowed the fewest grounders in all of baseball — 1570, one below the Marlins. Francisco Lindor is a tremendous defender at shortstop. He isn’t quite Simmons-level out there, but he isn’t far behind. Watch this pure nonsense, for example: In a down defensive year in 2019, Lindor was worth 0.033 runs above average per chance. For his career, he’s posted single-season numbers between 0.02 and 0.05 runs above average per play every year. Again, not Simmons, but still excellent. Next, let’s take the case of Paul DeJong. He also had a good 2019 with the glove, saving 0.035 runs relative to average per fielding play. That number is close enough to Lindor’s that the two are more or less indistinguishable. And yet, in only 14% more innings in the field, DeJong was worth 65% more runs above average on defense. That sounds crazy! The two were more or less equivalent defenders when a ball came their way, but the number of chances just adds up. DeJong only played 14% more innings in the field, but he had 51% more chances than Lindor. This wasn’t some fluke of positioning either, where intelligent placement brought DeJong into the action more often. The Indians finished second-to-last in baseball in the share of batters faced that hit grounders, while the Cardinals finished sixth-highest. If you’d prefer the raw number of grounders (defensive value is a counting statistic, after all), the Indians were last while the Cardinals were eighth-highest. The Rockies induced the most grounders in baseball (and also faced the most batters), which made Trevor Story and Nolan Arenado, who hardly needed the help, look even better. But before you get worried that there’s no point to using these stats because opportunities differ so much, consider this: the total effects aren’t massive. If Lindor had faced a DeJongian level of grounders, his defensive value would indeed increase — by around 2.5 runs saved. That’s far smaller than the level of uncertainty around defense, a small enough number that it doesn’t overcome random noise. If this gap in opportunities persisted for 10 years, with Lindor and DeJong playing the same level of defense behind vastly different pitching staffs, it would work out to less than three wins above replacement over the decade. But it does help to explain variation within a year. You might look askance at a defensive metric that says Paul DeJong is much better than Francisco Lindor, because Lindor looks so smooth out there. But the two were nearly equivalent this year in terms of converting chances into runs saved. There are many reasons not to rely too heavily on single-season defensive metrics. The sample sizes are small, and year-to-year correlations aren’t high. But chances matter, too. They can’t turn a bad defender good, or a good defender bad, but they can make two similar defenders grade out differently. It’s not the biggest effect, and it’s not even enough of an effect that two teams should swap otherwise equivalent players to maximize their skillsets — making a trade for the sake of a few runs per year seems silly, even for a numbers nerd like me — but it’s something to consider when you’re perusing stats in the offseason. And if nothing else, it was a great excuse to watch video of two great defensive plays.