More Plate Coverage Isn’t Necessarily Better

Writers, players, and managers alike often invoke the idea of “plate coverage” as a holy grail. See for example Mike Axisa’s take in 2016 on how Trevor Story’s plate coverage fueled his impressive start. Here’s one from Dan Connolly in 2015 about the then-hot Jimmy Paredes and his impressive plate coverage. And in 2014, Harold Reynolds and Dan Plesac produced a whole video on the issue for MLB Tonight.

Given these articles and similar widely held beliefs about plate coverage, you could be forgiven for thinking more plate coverage is better. And why wouldn’t it be? A hitter who can reach more pitches should be a more dangerous hitter, because they have more opportunities for contact.

But I was curious. Is this conventional wisdom true?

Methodology

To find out if more plate coverage is indeed better, I:

• Examined Gameday data for all balls hit fair from 2015 – 2017 by non-pitchers with at least 50 such balls hit. I didn’t count foul balls because to me, a foul ball doesn’t achieve the goal of plate coverage, which is to put the ball in play. I also didn’t count swings; those represent attempted plate coverage. I wanted to focus on actual plate coverage.
• Defined horizontal coverage as the median absolute deviation (MAD) along the x-axis and vertical coverage as the MAD along the z-axis (which is what Gameday defines as pitch height off the ground). Statcast gives this data in feet but for this article I’ve converted them to inches, because baseball is a game of them.
• Computed plate coverage in square inches by multiplying both MADs together.

Example: Plate Coverage in 2016

The following graphs show the worst and best horizontal plate coverages of 2016:

Delino DeShields can really cover pitches inside. Here’s one of them he got a handle on:

 

It’s a bunt, yes, but it’s still impressive coverage. But although he handled inside pitches well, the distribution of balls he was able to hit fair, along the x-axis anyway, was only 4.5 inches. That’s half the horizontal coverage of Caleb Joseph, whose distribution was twice as wide: 9.1 inches. The boxes in the boxplots tell the tale; DeShields’ box is much narrower than Joseph’s.

How about vertical plate coverage from 2016?

Corey Dickerson’s vertical coverage stretched from 0.8 feet off the ground to 4.5 feet off it, whereas Taylor Motter’s coverage extended from about 1.5 feet off the ground to 3.5 feet off it.

Combining both horizontal and vertical coverage gives us total plate coverage, in square inches. The leaders in this metric for 2016 were:

Joseph and Dickerson lead the list, as does free-swinger Salvador Perez. (Former) teammates Matt Wieters and Adam Jones, another free-swinger, come in next. Except for Joseph, these guys are all full-time players. Their bats can reach a large variety of pitches in the strike zone.

When thinking about plate coverage, consider that a baseball is about 2.9 inches in diameter. Joseph’s plate coverage of 81 square inches means he can hit and keep fair a baseball whose center of mass can be in about 28 different locations. Jose Abreu’s plate coverage of 66 inches means he can reach a baseball in about 23 different locations. Pitchers have more room to miss their spot with Abreu than they do with Joseph. These numbers aren’t literal; we’re looking at median absolute deviation and there’s a continuous range of baseball locations. But the general idea holds.

Here are the 2016 laggards:

This list contains up-the-middle players galore: Alex Avila, Josh Phegley, Greg Garcia, Francisco Cervelli, Tommy La Stella, Nick Hundley, and Delino DeShields. Motter is a utility player but has spent most of his time at shortstop. None of these guys are particularly good hitters. Now we may have an idea as to why. Pitchers have an easier time with these guys because they can’t reach as many balls. The guys in this list mostly rely on their defense to stick in the majors. Alex Avila’s coverage fits about 8.6 baseballs in it.

Comparing league-worst Avila to league-best Joseph, we see the differences in plate coverage:

Joseph clearly was able to hit, and keep fair, much wider range of pitches than Avila did.

Is More Plate Coverage Better?

But so what? Is increased plate coverage a good thing? The following graph gives us an idea:

The graph hints at an answer: “up to a point”. See how wRC+ rises along with plate coverage until the latter reaches about 50 square inches, then declines as coverage further increases. With an R2 of .019 vs. an R2 of .003, a parabola is a better fit than a linear model (although neither indicates plate coverage has a large effect on offense).

This answer makes sense. With too little coverage, pitchers can easily sneak pitches past you. With too much coverage, you put balls into play that you shouldn’t. There’s a sweet spot in the middle.

Recall the pitch DeShields hit, above. It was a clear ball, but he made contact with it anyway. Now see the following pitch to Caleb Joseph. It was 10.6 inches off the ground, the lowest pitch in 2016 that he hit fair, but he swung anyway:

 
That’s, er … not a pitch you should swing at. To be fair, there were runners on the corners with no one out in the seventh inning with the Orioles down by two. I’d be aggressive in that situation too!

Where’s the Sweet Spot?

The scatterplot above indicates the sweet spot is around 51 square inches. With this in mind, let’s see who had the most optimal plate coverage in 2016:

These hitters averaged a 92 wRC+.

Now those with the least optimal plate coverage in 2016:

Note this list is a mix of 2016 players with both large and small coverage areas. These hitters averaged an 88 wRC+, 4% worse than the group with optimal coverage.

Year-to-Year Changes

Now that we can estimate the optimal plate coverage for a hitter, we can see how year-to-year changes affect offensive performance:

On the x-axis, positive numbers indicate getting closer to optimal plate coverage. For example, a point at 10 could indicate the player moved from 30 square inches of plate coverage in 2016 (an optimal distance of 21 square inches) to 40 (optimal distance of 11 square inches) in 2017. Conversely, negative numbers indicate a player moved further away from optimal plate coverage.

The trend line indicates that the players who move closer to optimal plate coverage tend to see their offense increase. This trend supports the idea that more plate coverage isn’t always a good thing; rather, moving towards optimal coverage at around 51 square inches is. Of course, I’m not controlling for factors such as age, but the general idea seems to pass the sniff test.

Since last year, Jose Peraza’s expanded his plate coverage by over 30 square inches and in the process moved further away from the optimal area. His wRC+ has dropped 44 points.

Although it seems to be just one pitch, Peraza’s expanded his strike zone up.

Meanwhile Adam Jones has shrunk his plate coverage by 24 square inches. This shrinkage moves him closer to the optimal coverage area. His wRC+ is down only 3 points.

So far this year, Jones is either laying off high and down-and-in pitches or has been unable to put them in play. From what we’ve seen about plate coverage, this change may be beneficial.

Conclusion

Caveats abound here. Studying plate coverage doesn’t produce actionable advice. You can’t tell a player “You need to cover 10 more square inches of the strike zone.”

Further, we haven’t identified a causal relationship. Does a sub-optimal plate coverage cause a lower wRC+, or do players with less offensive prowess attempt to cover a sub-optimal area? Also, this study doesn’t control for age. A future study might get more granular by looking at xwOBA, instead of wRC+. Finally, I’ve used data to identify a major league-wide optimal plate coverage. Each player is bound to have his own optimal area, based on his skill set. This area may even vary by game situation.

But overall, studying plate coverage can give us a small bit of insight into why a player might be struggling at the plate. We now understand that more coverage isn’t always better. What matters more is how close the player is moving to the optimal coverage.