Steven Kwan, Geraldo Perdomo, and the Victor Robles Problem

Sorry, but this is going to be kind of a bummer. Our topic today is the crushing weight of statistical determinism. In researching this article, I learned something that increased my knowledge but also decreased my sense of the possible, and it made me a little bit sad. I would now like to share my sadness with you. We’re going to be studying the Victor Robles Problem.

You might not remember the days when Victor Robles was a star prospect. After short, impressive stints in 2017 and ’18, he had a breakout season in 2019, putting up a 92 wRC+ and 3.5 WAR, and finishing sixth in NL Rookie of the Year voting. ZiPS projected him for 3.3 WAR in 2020. If he could take the next step offensively, he’d be a star; if his offense remained just a bit below average, he’d still be a very productive center fielder. Instead, he turned in three straight seasons with a wRC+ under 70. Here are the Statcast gearboxes for his rookie season and 2022:

There’s a whole lot of blue in the top two rows. Plate discipline was the concern when Robles was first called up, and that was certainly an issue, but the lack of power stands out much more. Although his max exit velocity indicates that he has the capacity to hit the ball hard, Robles’ average exit velocity has been in the first percentile in each of his big league seasons, and his hard-hit rate has never been better than fifth percentile. The Victor Robles Problem is a question: Can a player who didn’t hit the ball hard as a rookie ever turn into a good hitter?

Here’s a quick refresher on why hitting the ball hard is a good thing. There have been 620 players with at least 400 balls in play in the Statcast era. They’re bucketed in the table below by their average exit velocity (EV) in increments of 1 mph. The first line shows average wRC+, and the second line shows the percentage of players with a wRC+ of 100 or higher”

It’s very difficult to be a good hitter if you don’t hit the ball hard. I pulled data for every rookie since 2015 and compared it to their stats in ensuing years (hereafter referred to as their veteran stats). As a note, rookie stats include a player’s entire rookie eligibility. For example, Robles’ include his cups of coffee in 2017 and ’18.

As I often do, I started by looking at correlation coefficients. It’s a quick way to get a sense of what’s connected, as well as how sample size is affecting the data. Below is a table that shows the correlation between rookie performance and veteran performance in three stats. It also splits them into buckets with a different minimum number of ball in play events as a rookie. Each group had a minimum of 100 BIP as veterans:

You’re likely not surprised that exit velocity and hard-hit rate stabilize quickly, or that they’re more self-predictive over time than wRC+. However, you might be surprised by the strength of the correlation. If a player puts at least 300 balls into play as a rookie, we can predict their future hard hit rate with a shockingly high degree of accuracy. The average player increases their hard-hit rate by a bit more than 1% after their rookie year. If we limit ourselves to players who had at least 300 BIP as rookies, only one player has ever increased their hard-hit rate by more than 10%: Vladimir Guerrero Jr., whom the whole baseball world expected to improve upon his disappointing (though certainly not bad) 106 wRC+ as a 20-year-old.

I don’t know about you, but I find the chart on the right a bit depressing. It’s good news for the Aaron Judges of the world, but it certainly doesn’t offer much hope to the Nick Madrigals among us.

Now it’s time for the big question. We’re looking at how a player’s rookie hard-hit rate correlates with their overall batting skill as a veteran. We’ll take the same rookie stats (and add max exit velocity) and check their correlation to a player’s wRC+ as a veteran:

EV stats and wRC+ have a similar correlation even after just 40 BIP, but EV stats improve more dramatically as the sample gets larger. If you want to know how well a rookie will hit in the future, you might be better off ignoring how good they are now and looking solely at how hard they hit the ball. Again, this might not surprise you, but it does illustrate the stickiness of EV and hard-hit rate, and the extent to which they affect wRC+ over a longer sample.

Now that we’ve determined the importance and immutability of exit velocity, it’s time to gentle ourselves to the icy touch of numerical certainty and condemn some of this year’s rookies to the realm of eternal weak contact. Here are the rookie stats for Geraldo Perdomo and Steven Kwan:

Perdomo and Kwan had very different seasons, but they have a few things in common. They’re rookies who played all year (and in Perdomo’s case, a bit of 2021), and hit the ball very softly. Both were pedigreed prospects who came into the season with questions about their power (though Kwan’s hit tool was classified as elite). What jumps out at me is that Kwan’s wRC+ is twice Perdomo’s, even though Perdomo’s hard-hit rate is significantly higher. Striking out twice as much doesn’t help, but there’s a second factor at play here. Perdomo had a higher hard-hit rate, but their EV was identical. Here’s how that’s possible:

Kwan hits the ball hard even when he doesn’t hit the ball hard. This helps explain why his BABIP is 74 points higher than Perdomo’s. When Perdomo doesn’t hit the ball hard, his EV is only slightly better than the 75.3 mph that Victor Robles put up as a rookie. Turning into Victor Robles is definitely not the way to solve the Victor Robles Problem.

On balls that aren’t hard-hit, Kwan’s EV is much higher, as are his wOBA and xwOBA. Kwan led the league with 99 balls hit between 90 and 95 mph. League-wide, those medium-hit balls have a wOBA of .269, nowhere near the .649 of hard-hit balls, but certainly much better than the .204 of balls hit below 90 mph. It’s definitely better to hit the ball hard, but it’s still helpful to avoid hitting the ball weakly.

Kwan shares his tendency toward medium contact with the very short list of players who have hit well as veterans despite very low hard-hit rates as rookies. From 2015-21, there were 75 rookies with a hard-hit rate below 28% (minimum 40 BIP). Only nine of those players went on to have a veteran wRC+ above 100. On balls that weren’t hit hard, all nine had an EV above the league average of 80.7 mph:

Some of those players, like Max Muncy, saw large jumps in their overall EV stats. They had high medium-hit rates as rookies not because they weren’t strong enough to hit the ball hard, but because there were a lot of balls that they just missed crushing. A select few players, like Arraez, Kemp, and Narváez, don’t have that kind of power, but have been able to succeed as veterans by relying on medium-hit balls. Unfortunately, that’s a very tough tightrope to walk:

The first row reminds us that it’s better to hit the ball harder. The second row says that even if you’ve already crossed the threshold into hard-hit territory, every last bit of velocity makes a difference. It’s the last row that isn’t quite as intuitive.

Over a short sample, players with a high EV on non-hard-hit balls tend to do better. However, when we increase the number of balls in play required to be in the sample, what tiny correlation there is turns negative. Players who max out with medium-hit balls don’t do well over the long-term. That kind of ball can fall in for a single, but it requires batted ball luck, which evens out over time. No matter the sample size, the correlation between a player’s medium-hit rate as a rookie and as a veteran is .31, much weaker than hard-hit rate. If reaching base that way is a repeatable skill, the only current players who really seem to possess it are Arraez and Kemp.

Among this year’s rookie crop, Kwan seems like the best candidate to join that club, along with Baltimore’s Terrin Vavra. Vavra had a non-hard-hit EV of 83.5 mph and a wRC+ of 97, despite a hard-hit rate of just 23.6%. However, if Kwan and Vavra don’t increase their overall power, they’ll always be more dependent than most players on batted ball luck. Kwan’s wOBACON was 37 points higher than his xwOBACON, the 17th-highest difference in the league, so he could be due for a visit from the regression monster. As for Robles and Perdomo, it might be time to leave them to the cruel hands of fate.

I would love to believe that I’m wrong here, and that players have a better chance of increasing their EV and hard-hit rate than the numbers indicate so far. We’ve only got eight years worth of exit velocity data, and further patterns might emerge after a few more seasons. This is also an era when players have more tools than ever to refashion their offensive profiles. Today’s players are receiving more instruction at the big league level and have access to outside instructors and hitting labs, so there are more resources for a player to rework their swing and unlock more power. For now, though, there doesn’t seem to be a great answer to the Victor Robles Problem.

Thank you to Mike Petriello, who encouraged me to research this topic with a text that began, “i have a story idea for you!” and who almost certainly didn’t expect me to run with the Victor Robles Problem as a title.