Who are baseball's most improved hitters so far in 2018? The fun part of that question is that there's no one official method to point to. You might like to see who has hit for more power, improved their plate discipline or simply piled up more hits. There's no wrong way to do this.
Things like batting average or home runs or walks, however, are results. That's fine, but what we like to do is to look for the underlying skills that get you there. Who has made more contact? Who has made better contact? And shouldn't you get credit for the skill shown in crushing a baseball at a velocity and angle that usually turns into a hit, even if it became an out because of a great defensive play?
That's the way we think of our "Expected wOBA" metric, which folds in all of that. (Read more about how it works here.) It looks at the combinations of strikeouts, walks, exit velocity and launch angle, and outputs a number that's similar to OBP, just with more credit given for extra-base hits. (The 2018 Major League average is .331.)
We'll look at the 208 hitters who had 100 batted balls both in 2017 and '18. That's actually slightly more than the 162 hitters who are "qualified" by traditional metrics, which gives us a larger sample. (Matt Davidson hasn't done either, if you're wondering where he is.)

Here are your 10 most improved hitters so far -- and our best guess at a reason why.
Mookie Betts, Red Sox
+.172 (up from .341 to .513)
Likely reason: Health/approach
Even though Betts is on the disabled list, this shouldn't be a surprise at all. Betts had a decent year in 2017, hitting a slightly above-average .264/.344/.459, but he was off to a monster start in '18, carrying a .359/.437/.750 line onto the DL. You're probably thinking this is just about being more aggressive, since that was the talk all offseason -- that new manager Alex Cora wanted his hitters to attack more hittable pitches.

It's a good strategy, and there's some truth to it. Betts is swinging at 60 percent of pitches in the zone this year, up from just 52 percent last year. But don't forget, Betts admitted that he played through a thumb injury for most of the second half of 2017, and it clearly affected him. He was hitting a strong .280/.356/.490 through the end of June, and just .248/.332/.427 after that. Healthy, at least before a recent abdominal strain, we saw what he can do.
Xander Bogaerts, Red Sox
+.100 (up from .286 to .386)
Likely reason: Health/approach
Speaking of Red Sox hitters, Bogaerts' rebound mirrors that of his teammate. Like Betts, Bogaerts wanted to get more aggressive this year, and he has, swinging at more than 61 percent of strikes after only 54 percent last year. Like Betts, he's hitting far fewer grounders, down to 40 percent from 48 percent last year. (Betts has dropped to 32 percent, from 41 percent.)
Perhaps most importantly, Bogaerts played through pain last year too. He was hitting a very good .308/.361/.455 when he was hit by a pitch in the hand on July 6, and just .232/.321/.340 afterwards. He admitted this spring that the injury had affected him. Like Betts, a healthy Bogaerts has been one of baseball's most improved hitters.

Francisco Cervelli, Pirates
+.087 (up from .325 to .412)
Likely reason: Health, probably
Cervelli has legitimately been one of the 20 best hitters in the game this year, so yes, he's improved from last year's below-average showing. It's simple to point to improved health here, since Cervelli was on the DL four times last year -- twice for a concussion, once each for a wrist and a quad. Maybe that really is all that it is.
It's worth noting, however, that in addition to a massively improved hard-hit rate (up from 32 percent to 45), Cervelli has also hit the ball hard in the air at a new level, unlike almost any other hitter. It's an open question as to whether this can last, but so far, so good.

Daniel Robertson, Rays
+.085 (up from .274 to .359)
Likely reason: Swing change
Sometimes, a swing change can mean everything. That's how the 24-year-old Robertson explained his breakout season when he was asked by FanGraphs in early May, saying that, "I've kind of bought into launch angle and getting on plane with the baseball." He went to the right coaches last offseason, seeking out the same swing gurus who helped J.D. Martinez.

Robertson has also improved his plate discipline, dropping his strikeout rate from 29 percent to 22, and upping his walk rate from 11 percent to 16 percent. His .263/.392/.437 is a big part of why the surprising Rays have hung around .500 despite a decimated rotation.
Michael Brantley, Indians
+.075 (up from .348 to .423)
Likely reason: Health
This one is easy! Of course it's health. No one's doubted Brantley's skills, but in recent years, he's been so often unable to stay on the field to show them. Last year, he attempted to return from multiple shoulder woes, and got off to a good start, even making the All-Star team. But then Brantley injured his right ankle in August, ultimately requiring surgery in October.

So far in 2018, only one hitter has struck out less often than Brantley's 8.6 percent. You'll find the only hitter ahead of him later on this list.
Amed Rosario, Mets
+.073 (up from .233 to .306)
Likely reason: Unclear
This is going to be the controversial one, because Rosario is still a below-average hitter. His 2017 debut was poor, as he hit just .248/.271/.394. Rosario's '18 season so far hasn't been much better, as he's hitting only .251/.284/.366. So how can we say he's improved?

Part of it is that the bar was set pretty low by Rosario's debut, but it's also because we're seeing positive steps in pretty much every important number. He's cut his strikeout rate from 29 percent to 21 percent. Rosario has doubled his walk rate from 2 percent to 4 percent. His hard-hit rate has jumped from 25 percent to 36 percent. These are all good things, especially for a highly-regarded prospect who is still only 22.
So why hasn't the production followed? It's not immediately clear. But there are at least positive signs here.
Mitch Haniger, Mariners
+.071 (up from .331 to .402)
Likely reason: Health / Swing change
Haniger (.268/.354/.500) missed time last year with a strained right oblique and also after being hit in the face by pitch, so this is somewhat a story about health. It's also about a player trying to improve, as he told MLB.com in May that "instead of hitting low line drives this offseason, I tried to elevate a little bit more."

But Haniger has also had a bit of an up and down year, hitting 10 homers in April and only three since, while having somewhat of a Rosario-esque situation where his underlying metrics have improved by a lot while his surface stats haven't. (Haniger was quietly pretty good last year, too.)
It's notable that Haniger has upped his hard-hit rate from 35 percent to 48 percent, and that he increased his walk rate from 8 percent to 11 percent, all while keeping his strikeout rate steady.
[tie] Matt Kemp, Dodgers
+.063 (up from .358 to .421)
Likely reason: Health/Conditioning
Kemp's unlikely resurgence (.349/.380/.589) has been one of the biggest stories of the year for the Dodgers, but the surprise here isn't so much his hot start as it is that he's managed to maintain it this long. We wrote about this in April, pointing out that as easy as it might be to dismiss Kemp's claims of weight loss, no one at the time had added more speed year over year.
That's still true, mostly. (He's not No. 1 any longer, but still in the top five.) It appears that Kemp's dedication to conditioning has paid off, as well as the fact he has managed to keep his lower half healthy -- he's got ankle surgery and five different hamstring pulls on his resume.

[tie] Andrelton Simmons, Aledmys Diaz,  A.J. Pollock
+.063
We featured Kemp, but he's really part of a four-way tie here at the bottom of our top 10.
Simmons, who recently went on the DL with an ankle injury is currently running the lowest strikeout rate (4.1 percent) of any qualified hitter since Tony Gwynn in 1998. Long known for his elite glove, he's now out-performing stars like Andrew McCutchen and Giancarlo Stanton at the plate. 
Diaz, who had a walk-off hit for the Blue Jays on Thursday night, is hitting the ball much harder than last year, upping his hard-hit rate from 24 percent to 41 percent, but he's also seen his BABIP drop from .282 to .210, which has limted his overall line. 
Pollock, meanwhile, had recovered well from groin and elbow injuries in recent years before hurting his thumb on a dive in May, simultaneously upping his hard-hit rate while lowering his ground ball rate.
Just missed: Javier Baez, Joe Panik, J.T. Realmuto, Yasmani Grandal, Daniel Descalso, Nick Markakis