Bryce Harper was as good as Mike Trout, until he wasn’t. It hasn’t yet been that big of a deal, with the Nationals up in first place, but Harper has been slumping, and the slump hasn’t been short. For weeks on end, he’s hit barely .200, and though the walks have still been there, Harper’s supposed to be better than this. He’s supposed to be one of the best, actually. That’s what he just looked like, at 22 years old, and instead now he’s a 23-year-old in a lineup being carried by Daniel Murphy and Wilson Ramos. To be clear, there’s absolutely nothing wrong with being on track for a four-win season. It’s just not how you want to follow a nine-win season.

There are plenty of indicators to point to. What happened to Harper’s numbers? His BABIP is a lousy .237. That’s guaranteed to come up. More discouragingly, he’s making more contact against pitches out of the zone. Last year, 70% of Harper’s batted balls came against pitches within the strike zone. That ranked him in the 67th percentile. This year he ranks in the 18th percentile. That partially explains why Harper’s exit velocity has dropped — and it has indeed dropped. That’s another thing. Harper so far has lost a tick or two on average.

Yet there’s still more. We all figured that Harper’s 2015 dramatically changed his own baseline. What if it shouldn’t have?

Just to let you know now, this is all going to be speculative. And there’s a pretty decent chance you’ll find it all unsatisfying. Still, I have to follow the numbers, and if I’m going to be honest, there was something that sort of troubled me about Harper’s 2015 performance. Were it not for Statcast, I would’ve loved it. And I still did love it, but Statcast made me question some things. Per usual, a lot of this information is coming from Baseball Savant. Last season, Harper — obviously — ranked in the 99th percentile in terms of slugging percentage on fly balls, line drives, and pop-ups. Air balls, basically. Yet in terms of average air-ball exit velocity, Harper was good but not elite.

Let me show you what this looks like. I looked at every batter with at least 100 batted balls captured by the Statcast system. In this plot, you see their air-ball slugging percentages against their air-ball exit velocities. I’ve highlighted Harper in red. His name is also inserted because why not?

There’s a very clear relationship, which only makes sense. Harder contact is better contact. Using the equation for the best-fit line, it’s easy to calculate “expected” air-ball slugging percentage. Harper actually slugged 1.238; based just on the batted balls, he would’ve been expected to have slugged .876. There’s a difference between those two numbers of 362 points. That was far and away the biggest such difference for any hitter in the pool.

All right, so what? What are we supposed to make of this? All this analysis remains in its infancy. But let me now show you the same plot as above, only this time with the 2016 data instead. Harper, again, is highlighted in red.

Harper now is pretty much exactly on the line. You can see the dotted line passing right through the red Harper data point. Now, again, Harper’s air-ball exit velocity has dropped somewhat, and that’s of some concern. He hasn’t been hitting the ball exactly the same. But where last year he blew his expected production away, this year he’s where the numbers think he ought to be.

Is there something special about under- or over-achieving? I took the 2015 differences between actual and expected slugging, then I plotted them with the 2016 differences.

There’s the hint of something there, but it’s mostly randomness. And keep in mind, all the numbers are unadjusted, like for, say, the ballpark. A hitter with a big positive difference in 2015: Carlos Gonzalez. A hitter with a big positive difference in 2016: Carlos Gonzalez. You have to figure that’s in part capturing the Coors Field effect. He’s the most upper-right gray dot. Chris Davis has posted consecutive strongly positive differences, but maybe that’s in part capturing Camden Yards. Again, I don’t really know! But the numbers are pretty convincing.

Think of it this way. The plot above includes all the players with at least 100 tracked batted balls in each of the last two seasons. Among them, the 20 players with the biggest positive differences in 2015 averaged a slugging difference of 170 points. Those same players have an average difference this year of 13 points. Meanwhile, the 20 players with the biggest negative differences in 2015 averaged a slugging difference of -144 points. Those same players have an average difference this year of -38 points. This doesn’t look like something you want to regress all the way to zero, but heavy regression appears to be called for. Moving forward, that would be bad news for Jake Lamb.

And it looks like it’s already been bad news for Bryce Harper. Part of the problem is that he’s making worse contact. I’m sure he knows that, and I’m sure that’s something he’s working on. But a big component here could just be us having made too much of the 2015 breakout. For whatever reason or reasons, Harper last year maximized his air-ball contact results, and it doesn’t look like we should’ve expected that to continue. This year, he looks more normal, mostly because his air balls are behaving more normally. It’s unsatisfying because we all hate the game being reduced to fluctuations in luck, but I don’t know of another takeaway. I’m open to suggestions.

Bryce Harper is an extremely good player. And it should be said that he has an extremely high power ceiling. I mean:

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We’ve seen the mammoth dingers. There’s no faking those. Harper has always been powerful, and he remains unusually young. There’s more in there for him to tap into. It just looks like Harper can’t hit the way he’s hit and expect 2015 results to follow. For those to happen, he has to hit the ball harder. I won’t pretend he can’t do it.