For Your Enjoyment, a Groundball Rate Refresher

Last week, in a bit of a horror story for pitchers, I demonstrated that they have little control when it comes to suppressing HR/FB rate. That’s quite depressing — if you face a big, strapping boy of a hitter, the fly balls aren’t likely to stay in the yard, no matter who you are. It’s enough to make you sad.

But rejoice! Baseball is more than just what percent of fly balls leave the yard. In fact, it’s a lot more than just that. For one, you could just strike people out. It’s hard to hit a home run if you don’t even hit the ball. Short of that, you could just induce a grounder. Unless the aerodynamics of the baseball and also the rules of baseball change markedly, no one’s hitting any home runs on the ground.

Intuitively, pitchers can do a lot more to control groundball rates than home run rates on fly balls. For one, name a pitcher who’s really good at suppressing home runs over a long career. I’m talking really good, not just kind of good. Did you come up with Adam Wainwright, Justin Verlander, and Clayton Kershaw? They’re the three best at it with enough innings pitched for the data to look meaningful, and even then they’re only a few percentage points better than league average.

On the other hand, it’s easy to name groundball pitchers. Zack Britton is the archetypal example, but Marcus Stroman, Dallas Keuchel, Charlie Morton, and plenty of others come to mind as well. Those guys may not do a great job of limiting home runs when opposing batters put the ball in the air, but they limit overall home runs all the same.

Still, you can never really be sure what’s skill and what’s noise. After all, it sure feels like a pitcher with a high home run rate is doing something bad to give up those dingers, and that doesn’t appear to be true. To get to the bottom of this, I investigated groundball rates in roughly the same way I previously looked at home run rates.

To recap, I used a divide-and-conquer methodology. I used 2018 groundball rates to separate both pitchers and batters into four quartiles of groundball rates. For this study, to ensure uniform definitions, I defined a grounder as any batted ball that had a launch angle below eight degrees. This disagrees slightly with the official count of grounders, but it’s less subjective than using what the scorecard has for each game, so I decided it was worth the effort to set a quantitative cutoff.

As before, I removed pitchers from the 2018 batting pool. From there, I looked at how each batter fared against the four groups of pitchers in 2019, and vice versa. To avoid representation bias in the groups, I weighted each group by the minimum of a batter’s four groups — Whit Merrifield, for example, faced 108 lowest-grounder pitchers, 136 in the second group, 160 in the third group, and 103 highest-grounder pitchers, so his weight in each quadrant is 103 batted balls.

Below, you can see what that looks like. The first line is what we would “expect” to see if pitchers kept the same groundball rates from 2018 to 2019 — if their rates were meaningful, in other words. I put the year in quotation marks because it’s not actually 2018 data — it’s the batter-pitcher matchups from 2019, but the pitcher groundball rates from 2018. The second line is what actually transpired in 2019:

That looks pretty good! The average groundball rate overall in 2019 was 42.9%, which means that we see slight regression towards the mean in every group, but pitchers who induced a lot of grounders in 2018 continued to exert their groundball tendencies in 2019.

It’s not so simple as saying that pitchers are in charge, however. Take a look at the same data going the other way. When we separate batters into four quadrants and see how pitchers did against them, we get a very similar pattern:

Interestingly, batters’ year-one groundball rates do a nearly equally good job predicting how they’ll do in year two. We’ve found a skill where pitchers and hitters both exert control; where both sides have skill that matters heavily in predicting the outcome. That’s true in every batter-pitcher confrontation, of course — baseball would be a lot less fun if only one of the batter or pitcher mattered — but it’s certainly not true for every skill.

Of course, if both batters and pitchers are important for groundball rate, we should have more matchups with extreme outcomes. If a high-grounder pitcher faces a high-grounder batter, there should be a strong pull for groundballs. Conversely, if a fly ball pitcher faces a fly ball hitter, not much should be on the ground.

To show this in a table, I looked at the results of each quadrant of hitters going up against each quadrant of pitchers. In the table below, the hitter groups are on the left and the pitcher groups up top:

To me, that’s a pretty compelling chart. Every step higher in groundball quartile in either cohort matters, and they all matter similarly. That’s grounders in a nutshell: everyone matters!

For comparison’s sake, let’s look at the same style of grid for home runs per ball hit in the air:

The difference is stark. In this table, going down a row and looking at batters who launch more home runs increases your home run rate, no matter where you start. Move to the right one column, however, to pitchers who gave up a higher rate of homers in 2018, and the needle barely budges. Sure, there’s a general drift, but it’s tiny and overwhelmed by the contributions of the batter.

Additionally, observed grounder rate seems to tell you something about a player’s true grounder skill very quickly. The spread in grounder rates from the top to bottom pitcher quadrants was 64% as wide in 2019 as it was in 2018. The spread in batter groups was 58% as wide, and in a smaller sample on average, which should tend towards more regression with all else equal. In other words, if you see someone hitting or allowing a lot of grounders, you’d be well served to believe it.

Like the home run rate finding, this isn’t earth-shattering. It’s something you probably know intuitively, something that scarcely needs elaboration. But there are a lot of ideas that you know that turn out to not be so. The earth was flat for a long time, obviously flat to any observer, until people figured that nugget out. I’m not suggesting that pitchers and batters having equal say in groundball rate is quite as monumental as the earth being round, but that doesn’t mean it’s not worth checking over the data once in a while to make sure we still know what we’re dealing with.