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How Paul Goldschmidt and Didi Gregorius both exemplify regression to the mean

Gregorius fell off his mountain, and Goldschmidt is finally turning things around.

Arizona Diamondbacks  v San Francisco Giants Photo by Ezra Shaw/Getty Images

At times this season, Didi Gregorius has looked like one of the best players in baseball and Paul Goldschmidt has looked like one of the worst.

Gregorius, the AL Player of the Month for April, slashed .327/.421/.735 with ten home runs and 30 RBIs in 121 plate appearances in his first 28 games. People knew that Gregorius’ performance was not sustainable, but nobody could have expected what was on the horizon for his next month.

In his 131 plate appearances since April ended, Gregorius has slashed .177/.221/.226 with just one home run and six RBIs. After all the hype that surrounded Gregorius’ first month, his season slash line is .243/.317/.450, right around his career averages. He’s been so bad, in fact, that we should almost expect Gregorius to re-regress!

On the contrary, Goldschmidt has been the antithesis to Gregorius. Over his first 48 games and 203 plate appearances, Goldschmidt was slashing .198/.320/.355 with just five home runs and 30 RBIs. His slump went on for so long that some believed that Goldschmidt’s age might be playing a factor into his poor performance.

It didn’t last too long after that. Over the last two weeks, there’s been no better hitter than the Diamondbacks’ Goldschmidt, who is hitting an absurd .417/.493/.950 with seven home runs and 18 RBIs over his last 69 plate appearances. Like with Gregorius, Goldschmidt’s season numbers are slowly — but surely — working in the direction of his career average. He had a .675 OPS on May 22nd. On June 11th, just 20 days later, Goldschmidt’s .873 OPS is only 54 points below the .927 OPS he has posted over the course of his career.

As you can see in the above chart, Goldschmidt is clearly a superior hitter to Gregorius, but he did not have a higher season OPS than Gregorius until June 8th, excluding their Opening Day performances.

Early in the season, all the indicators showed that the luck for both would soon switch. Sometimes it never does, and that can be concerning. Most around baseball, though, expected it to happen.

Goldschmidt and Gregorius, by month

Player April wOBA April xwOBA April EV May wOBA May xwOBA May EV June wOBA June xwOBA June EV
Player April wOBA April xwOBA April EV May wOBA May xwOBA May EV June wOBA June xwOBA June EV
Paul Goldschmidt 0.383 0.387 90.6 0.241 0.306 86.9 0.683 0.638 95.8
Didi Gregorius 0.457 0.400 85.4 0.172 0.226 86.5 0.269 0.311 87.1

As you can see in the bolded figures, even during Goldschmidt’s horrible May, he still had a higher average exit velocity on batted balls than during Gregorius’ MVP-caliber April. The balls just weren’t falling for hits for Goldschmidt; nearly everything was falling for a hit for Gregorius.

Of course, after seeing all of this, there are larger lessons that can be learned.

The first lesson is to never let early slash lines deceive you. I think this lesson is an important one, even for myself. I was not completely fooled by Gregorius’ early play, but I do admit that I was looking for significant changes in his game. It is true that he was walking more during that first month; in fact, he has 24 walks already this season, just one shy of his entire 2017 total.

It is also true that he has taken advantage of Yankee Stadium’s dimensions, with nine of his 11 homers coming at home, all of which have been hit to the short fence in right field. In my mind, if Gregorius had maintained his improved discipline, he would continue to hit well, as Yankee Stadium would always remain his home park. That, for one, was not changing. Clearly, I was wrong.

Goldschmidt, too, raised some concerns from baseball writers, including from Jay Jaffe at FanGraphs, who recently wrote about Goldschmidt’s struggle with high velocity four-seam fastballs this season. That may still be an issue going forward, though it remains to be seen whether this will continue. For now, though, Goldschmidt is crushing and his luck may be beginning to even out.

The second lesson is to understand that baseball has a funny way of almost always regressing to the mean. It’s a sport unlike any other; there are so many games, so many innings, so many plate appearances over the course of the season that things are bound to even out according to actual skill. It is not until at least Memorial Day (if not later, like for Goldschmidt) that we can truly begin to evaluate whether players have made sustainable changes going forward.

Players’ lines are still evening out, and they will continue to even out as the season progresses. For now, though, Goldschmidt and Gregorius give us further evidence that baseball will always remain a marathon and not a sprint.

Devan Fink is a Featured Writer at Beyond The Box Score. You can follow him on Twitter @DevanFink.