Understanding Hitter Development

Trying to crack the code to quantify the development of young hitters

“Baseball is a very difficult game. You throw the ball, you hit the ball, and you catch the ball. Sometimes, it rains!”

Okay, so enough of the Bull Durham references. Case in point; for as any as athletes can make the game of baseball look, it is, in my opinion, the most challenging sport out there. Could you imagine trying to have to account for a upper-90s MPH fastball and then having to worry about a wipeout slider? Here I thought it was difficult playing high-school baseball!

In addition to being extremely challenging, baseball is also a skills-based game. Athleticism can get you to a certain point, but, at the end of the day, on-field abilities will in out. On the bright side, skills can be improved over time, so there is a lot of development that can take place with this sport.

We often talk about the idea that young players have the ability to improve at any time, but it is tremendously more complex than that. How much does the average young player improve? What types of players are more likely to improve? When do we generally see the typical player capped in terms of expected development? There are so many questions that we would benefit from knowing the answer to!

That will be what we are trying to accomplish today. By analyzing rookies who have come into the MLB since 2010, and their overall development, we can start to establish expectations for how young players should develop. In this piece, we will be specifically focusing on the hitter side of things, before turning out attention to pitchers down the line. Based on my general analysis, I would consider hitting to be the more skilled-based practice of the two- it is much reactionary than pitchers, who are in control of the pace of the game.

NFL fans, think about it this way: hitters are the cornerbacks facing the pitchers, who would classify as wide receivers. It’s a very daunting task to hit a baseball traveling 95+ MPH, and it only becomes more challenging with how talented pitchers have become. As we look for the next group of premier young hitters, which ones are most likely to take a sizable leap forward, and by how much? Let us dive right into it!


What Is The Rate of Development For Young Hitters?


Before we continue with the study, there are a few details to go over. As mentioned earlier, the sample size of this study is any hitter who debuted from 2010, but only those with at least 350 plate appearances in their rookie season and 400 plate appearances in the following years. Not all of the players with the 350 rookie plate appearances will qualify in subsequent seasons, though that, in its own self, is telling; outside of injuries, a lack of meeting the criteria does indicate that they weren’t productive enough to receive notable playing time.

Immediately, upon beginning this study, the first question I pondered was what is the learning curve for young hitters coming out of their rookie season. To use this, we will be using weighted-runs-created-plus (wRC+), a statistic that combines on-base percentage and power, weighting on-base percentage more heavily to capture each player’s offensive value, and then adjusting for ballpark and run environment. A 100 wRC+ is league average, while a 105 wRC+ would be 5% above league-average, with a 95 wRC+ being 5%  below league-average. The simplicity in how it is displayed makes it quite easy to understand, and it is, in my opinion, the one offensive statistic that is able to truly quantity a player’s offensive output.

With that, let us take a look at how much a player’s wRC+ improves from season to season! When do players take the biggest leap? Here are the average percentage increases in wRC+ from one year to the next:

Percentage Increase of wRC+

The third year is generally cited as being the “leap year”, and that appears to hold a significant amount of merit. On the surface, it would appear that while hitters can slightly improve in their second year, the major growth happens in their third season. Furthermore, by the fourth season, it would appear that the average hitter is generally “capped out” in terms of upside. Does that mean they won’t improve? Absolutely not! We have seen plenty of hitters make an unexpected breakout at various stages of their career. Yet, based on the expectation, you should not be expecting too much further growth from a hitter after a player’s third season.

The percentage increases may seem remarkably small, but this sample size includes hitters who hit their peaks in their rookie season, thus their wRC+ was bound to come down. If we put hitters into a specific bucket based on their wRC+, however, a different story is told.

Percentage Increase of wRC+ Based on Rookie wRC+

As you would expect, hitters with a very poor rookie wRC+ are more likely to improve. Remember, though, that even though they improve, it still may not be significant. After all, going from a very poor wRC+ to a poor wRC+ is much less meaningful than going from an average wRC+ to an above-average wRC+; unless you are a superb defender, the bar to be an everyday player is to be around a league-average hitter. At the same time, there are enough success stories out there that you cannot write a player off after one poor season:

Struggling Rookie Hitter Success Stories

On the other hand, of the 51 players with at least a 120 rookie wRC+ with at least three years of experience, it represented a peak season for 27 of them. There are certainly some cautionary tales to be concerned about:

Rookie Hitters Who Peaked Mightily

That being said, a majority of above-average rookie hitters will continue to be above-average hitters. It is the least significant season with regards to average wRC+ over the first six years of one’s career, though it is extremely important to look beyond just the wRC+; is their performance sustainable, or are they relying on a lot of batted-ball luck? Going underneath the hood can be critical in deciphering the star players from those that may have already hit their peak.

Really, none of this is particularly surprising; good hitters usually remain good, while there is room for below-average producers to improve, but to what extent. What is more complex, however, is when we break down the art of hitting into specific categories.


How Does A Hitter’s Power Develop?


The development of power is a very interesting case study. One on end, a lot of it is physical, so you would think it would come naturally. Yet, at the same, hitting for is as much about hitting the ball at optimal launch angles as hitting the ball hard, while players can develop physically more over time.

To measure power, we will be using isolated power (ISO). This is actually quite a simple statistic: taking a player’s slugging percentage and subtracting their batting average from it. See, one’s slugging percentage gives a boost should you accumulate a lot of singles, but that’s not hitting for power! For perspective, among hitters with at least 400 plate appearances, a .183 ISO was average, a .232 ISO was the 80th percentile, and a .129 ISO was the 20th percentile. Now, it’s not as simple as wRC+ in terms of how it is displayed (can we please get an ISO+?), but the simplicity of the calculation, along with its overall effectiveness as a statistic, makes it a clear go-to statistic for measuring power production.

In terms of the development of power, it is remarkable how similar it mirrors wRC+:

Percentage Increase In ISO

It appears to certainly take a few years for hitters to fully tap into their power. By year three, though, we start to see that power on full display. What is also notable, too, is that, albeit in a smaller sample, hitters past year three can continue to tap into more and more power. After all, almost every hitter is always looking to optimize their swing to its fullest capacity, and can always gain more physicality as time goes on. Meanwhile, the development of power is really divided based on what “power” bucket you fall under.

Percentage Increase In ISO Based on Rookie ISO

This continues to be in line with what we saw from wRC+, which isn’t surprising; if you hit for more power, your wRC+ is going to be higher. Remember, a strong amount of power development has to do with overall physical development and approach changes. If you’ve already maxed out physically and have a fully optimized swing, there isn’t much room for growth left in terms of your power. On the contrary, hitters who are still maturing physically (younger hitters) or improve to improve in that department.

Ironically, this is very similar to the “catch-up” line in economics; poorer countries, based on the economic growth theory, are expected to catch up the richer countries over time. Eventually, most lower-power hitters who have some growth left either physically or with their swing, are expected to close the gap over time. This certainly doesn’t apply to every hitter, but it is a good rule of thumb to follow. In fact, it becomes even more apparent when isolating the development of power for hitters to those who demonstrate numbers in their first season:

Hitters With Minimal Power in Rookie Year That Developed Power

3 responses to “Understanding Hitter Development”

  1. Benny the Jet says:

    Phenomenal article, well done! I nabbed Vaughn and Franco in my dynasty league 2 years ago. Really high on both of them and this article reaffirmed things. That being said, I have Kelenic and I’m still holding out hope, simply due to his plate discipline in the minors.

  2. krsfaz says:

    I’m curious to know what you think about Gavin Lux. Someone who was graded with 60/70 game power coming up through the minors, and a 60 grade hit too, came up to the majors and struck out close to 30% of the time with a close to average ISO, but has now cut that K-rate to below 20%, but has seemingly sacrificed any power (a putrid .071 ISO) for contact. Which group would you put him in?

  3. Jake Federman says:

    Awesome article. Did you think about binning the hitter metrics by number of plate appearances rather than year? By that I mean tracking a hitter’s wRC+ in PA 0-x, and then x-y, and then y-z, as opposed to his wRC+ in season 1-2, 2-3, etc.

Leave a Reply

Your email address will not be published. Required fields are marked *

Account / Login