Here Are Baseball’s Most-Changed Hitters

The other day, I wrote about Corey Dickerson, and how, since he’s joined the Pirates, he’s all but stopped striking out. I went through and tried to identify changes he’s made. This has become a fairly routine writing process, looking at changes in the numbers and then going to the video. I realize I write more about players who are becoming good, or who are becoming bad, than I do about players who are still good, or who are still bad. Players who stay the same are old news. I’m biased to look for players doing something different, and in that regard I’m not alone.

Players who change are interesting. Players who don’t change are interesting, too, but those players are just out there, acknowledged and understood. When players start making adjustments, or when opponents start making adjustments to them, it’s like you’ve got a mystery box, and you just want to know what’s in it. What is the new player going to be? Has he become truly better, or truly worse? Players become moving targets, and if nothing else, we can stand to learn from whatever happens.

I guess I don’t need to justify this. I think many of us are in the same boat. We want to know who’s changing, and how. I’ve run some math. This post will have a big table in it.

I decided to try to answer the question: Which hitters have changed the most? I examined the player pool of guys who’ve batted at least 100 times in both 2017 and 2018. The sample numbers 193. That’s roughly six and a half players per team.

This paragraph is going to explain the method. You can skip this if you like, because it’s going to be dense. I settled on four core traits: swing rate, contact rate, average launch angle, and average exit velocity. For each player, for each season, I calculated z-scores for these measures, showing the number of standard deviations above or below the mean. The penultimate step was to calculate the absolute values of the changes in the z-scores. Read that again if you have to, because there’s a lot going on. In this way, if someone’s z-score has gone down by 2, that counts the same as if the z-score has gone up by 2. Changes are changes, right? Doing that, I got four differences, one for each stat. Then I just added them up for a total. The higher the total, the more dramatic the change.

Before we get to the fun part, it needs to be said this isn’t all good. A player who’s changed for the better might be shown right alongside a player who’s changed for the worse. This is focused on changes for their own sake. And now, a table of the 25 most-changed hitters so far. The absolute values of the changes are summed for the total, but for the core stats, the table shows the direction of the change.

The 25 Most-Changed Hitters
Player Team Swing% Contact% LA EV Total
JaCoby Jones Tigers 1.2 1.3 1.0 1.3 4.8
Gregory Polanco Pirates -1.2 -1.5 1.5 0.4 4.6
Matt Davidson White Sox -1.3 0.9 -2.1 0.2 4.5
Matt Carpenter Cardinals 0.2 -1.8 -1.2 -1.0 4.2
Michael A. Taylor Nationals 0.5 0.4 -1.7 -1.6 4.2
Jordy Mercer Pirates 0.9 -1.0 1.3 0.7 3.9
Mitch Haniger Mariners 0.4 -1.1 1.3 1.2 3.9
Adeiny Hechavarria Rays -1.1 1.0 0.1 -1.5 3.7
Jonathan Villar Brewers 0.0 -0.1 -1.8 -1.8 3.7
Leonys Martin Tigers 0.0 0.7 1.7 1.2 3.7
Paul DeJong Cardinals -2.1 -1.0 -0.2 0.3 3.6
Lorenzo Cain Brewers -1.9 -0.3 -0.7 -0.8 3.6
Mallex Smith Rays 0.6 1.3 0.0 1.8 3.6
Nick Delmonico White Sox 1.0 -0.2 0.7 1.7 3.6
Robinson Chirinos Rangers 1.2 -1.5 0.5 -0.4 3.6
Chris Davis Orioles 0.5 0.4 -0.8 -1.9 3.5
Francisco Cervelli Pirates -0.2 0.1 2.3 0.8 3.4
A.J. Pollock Diamondbacks 1.1 -0.8 1.2 0.2 3.3
Joe Mauer Twins -1.2 -0.9 0.0 -1.1 3.3
Billy Hamilton Reds -0.3 -1.3 -0.4 -1.3 3.3
Corey Dickerson Pirates -0.5 1.4 0.9 -0.4 3.3
Yonder Alonso Indians 0.8 -0.1 -1.5 -0.8 3.3
Wilmer Difo Nationals -1.9 -0.2 0.6 -0.5 3.2
Howie Kendrick Nationals 1.3 -0.2 1.3 0.4 3.2
Robinson Cano Mariners -1.4 -0.9 0.4 0.4 3.2
SOURCE: Baseball Savant
Minimum 100 plate appearances in both 2017 and 2018. Changes in core statistics are expressed as z-scores, and then the absolute values are summed for the total. I know this sounds kind of complicated.

Dickerson sneaks in there toward the bottom, in 21st place. I know this method is far from perfect, and it leaves out certain things, but at least it passes the smell test. For Dickerson, what’s driving this is that his contact rate has improved by 1.4 standard deviations. That is, it’s better by nine percentage points. His launch angle has also changed by about six degrees. Dickerson isn’t what he was in 2017. Or, for that matter, ever.

Something I noticed pretty quickly is that Dickerson is not alone here. This table includes four different Pirates hitters, which is more than you would ever expect. It would be impossible for me to say whether that means anything about the team, but in a way, the Pirates lineup is sort of like an active experiment. Dickerson slots in at 21st. Cervelli is 17th. Mercer is sixth. Polanco is second. See, Cervelli is suddenly hitting everything in the air. Mercer is doing the same, and he’s sacrificed contact. Polanco is hitting balls in the air, and he’s sacrificed contact, and he’s swinging less often. The Pirates lineup might surprise some people, and that could even include the Pirates themselves. These are players in some state of flux.

Enough about the Pirates, though; this table is headed by a former Pirate, in Jones. We haven’t paid a whole lot of early-season attention to the Tigers, because the Tigers are kind of depressing. But Jones is actively trying to do something incredible. Last season, Jones struck out 42% of the time. Read that again. Jones struck out 42% of the time. That’s 42% of the time. If Jones had batted 100 times, then, by his results, in 42 of those times, he would’ve struck out. And now? Right now, Jones’ strikeout rate stands at 25%. His contact rate has improved by nine points. He’s hitting the ball hard. I don’t know if this version of Jones is an everyday baseball player, but for him to even be in the conversation is astonishing, given how he looked before. In the majors last season, Jones was hopeless.

And so was Davidson! Last year’s Davidson was worth a win below replacement. This year’s Davidson has already been worth a win above replacement. His contact rate is up six points. His swing rate is down more than seven points. Davidson, if anything, has leveled out his swing. I didn’t think it was worth it for the White Sox to keep Davidson around in a regular role, myself, but here we are. Davidson might be figuring it out.

As an example of someone who might not be better, Carpenter’s in fourth, and he’s there in large part because his contact rate has dropped by more than ten points. He’s also generating a little less loft, and while the league around him has apparently been hitting the ball harder, Carpenter hasn’t risen with the tide. Again, changing isn’t always a good thing. The Cardinals would like to have the Carpenter they had before. That guy was outstanding.

I’m not going to go over every hitter on the list. If you want to know more, you can click on their names. Or you can just wait, since odds are, if these changes keep up, pretty much every player will eventually earn his own front-page article. And for anyone curious, the least-changed hitter, at present, is Michael Brantley. He is just barely holding off Scooter Gennett. I have no further thoughts.

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Jeff made Lookout Landing a thing, but he does not still write there about the Mariners. He does write here, sometimes about the Mariners, but usually not.

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minor suggestion for presentation. while u took the absolute value to make the list, in the actual table in the article it is prob good idea to present the changes with +- included, so we could see direction of change for each attribute


Yeah, it looks like Jordy Mercer and Mitch Haniger are nearly identical to one another in their change this season, but in reality they’re complete opposites. Mercer’s effectiveness at the plate has deteriorated while Haniger has turned himself into one of the best hitters in the game so far. Calling them the same doesn’t really tell us anything useful.