Originally written on Fangraphs  |  Last updated 11/16/14
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Jeff Zimmerman and I have done lots of work on player aging curves in the past 12 to 18 months. Jeff started things off with a series of hitter aging curves, which focused mostly on standard outcomes and WAR components. Jeff and I then joined forces this year for a series focused on pitcher aging.
This time around, I wanted to know how a hitter’s plate discipline changes over his career. We already know plate discipline statistics are easily the most stable, year over year. That said, I wondered whether I’d see meaningful patterns as players age. Often times, scouts and commentators mention how a hitter’s approach changes over time: less disciplined, less contact as a young player; better bat control and better strike-zone awareness as a hitter matures. But does the data confirm this thinking?
For these curves, I focused on the standard plate-discipline statistics available through our PITCHf/x data — except for “pace.” I also included swinging-strike percentage from BIS. To calculate the curves, I used data from 2007 through 2012 and applied the same methodology that Jeff and I used for the pitcher-aging curves.
Here are the curves:

That’s pretty messy. To make things easier, I split the curves into swing data and contact data, which is shown below.
The first thing you should notice is there really isn’t a whole lot of change in these metrics as players age. This makes sense, given that these measures enjoy a high correlation, year to year. The biggest change we see relative to age 21 is for O-Contact%, which decreases almost 5% by the time hitters reach 30 years old. But outside of that single metric, none of the others varies more than 3% over time. What’s really interesting is that while O-Contact% drops dramatically, the impact on SwStr% is muted. That’s because while O-Contact% is dropping, hitters aren’t swinging at pitches outside the zone as often.
Let’s break up the data a bit and look at swing data first (I’ve left in the Zone% and Swinging Strike% for reference):

Overall, hitters generally decrease their swing rates during their careers. Younger players almost immediately increase their selectivity. In particular, hitters decrease their number of swings at pitches outside the zone. This aligns with the general idea that hitters become more selective the longer they are in the league. But the selectivity doesn’t increase forever. At about age 33, we begin to see a slight uptick in Swing%. Hitters seem to be swinging more at pitches both in and out of the zone.
Also, the older the hitter, the closer the trend in swing rate and Z-Swing% becomes. That’s largely due to the fact that pitchers appear to throw more pitches in the zone to aging hitters. There is effectively a “mini curve” for Zone% from ages 21 to 28, which happens to align with a hitter’s peak years. After 28, the percent of pitches in the zone increases by 1% through age 34—and almost 3% if they last until age 39. There are probably two things going on here, and it’s hard to parse out. On the one hand, as hitters decline, pitchers might realize they can get away with attacking the zone more often. In fact, we see a negative correlation between wOBA in year one and Zone% in year two (-.39). On the other hand, hitters become more disciplined and offer less frequently at pitches outside the strike zone during the same period—at least until age 33. So part of the effect may simply be that pitchers are forced to come into the zone more often against aging hitters because they are less likely to chase bad pitches.
Now let’s look at the contact data:

Here, the story is similar to the trends for swings. That is, with one huge exception: contact outside the zone. Hitters generally increase their contact rates through age 29. After that, contact begins to decrease, driven lower by the drastic decline of contact outside the zone. This coincides with the rise in swinging strikes, which also beings to ascend around age 29. This likely reflects the general aging of a hitter’s skills: Slower bat speeds force players to cheat more on fastballs, which leaves them more vulnerable to pitches outside of the zone.
For a practical example of how a player’s plate discipline changes over time, I took two players who are similar ages (Brendan Ryan and Josh Hamilton) where we have PITCHf/x data and plotted their O-Contact% and SwStr% since 2007 (general curves for both metrics are faded):

Given the plate appearance cut-off level I set for this (250 PAs per season), we have three data points for Ryan and four for Hamilton. Both start at age 27. Ryan and Hamilton both saw large decreases in their O-Contact%, which is consistent with the general curve. What’s different is that Ryan’s swinging-strike increase is consistent with the finding that big decreases in O-Contact% are not associated with similar size increases in swinging strikes. Hamilton, on the other hand, saw his swinging-strike rate jump at a much higher rate, relative to his drop in O-Contact%. What’s driving this is the difference how pitchers attack these very different hitters.
Since turning 27, Ryan has seen the percent of in-the-zone pitches thrown to him decrease by only 1%. Hamilton, has seen his Zone% drop by more than 6% (second most in the league) and by 5% in the past year. This makes sense, given that Hamilton is an infinitley more dangerous hitter than Ryan, and he plays his home games in a very hitter-friendly park.
Ryan has also cut down on swings at pitches outside the zone by 2%. Hamilton’s rate has jumped 5.7%; and 5.5% just this year. His SwStr% increase is the highest during the time examined here, and his O-Swing% increase is 12th worst.
This adds to the decision-making calculas for teams that might offer Hamilton a large contract in the offseason. The question her is whether Hamilton will adjust to the way pitchers approached him this year. Outside June, Hamilton saw fewer pitches in the zone as each month passed. Hamilton’s swing-strike percentage also increased every month — except July. Even them, it was only down .1% when compared to June. Hamilton had the toughest time in September and October, when he bumped his SwStr% more than 6% from August and swung at 4.6% more pitches outside the zone.
With that said, Hamilton still finished 2012 with a monster offensive season. Even in September and October, when his SwStr% jumped to more than 24% and his O-Contact% fell to only 38%, Hamilton still posted a 122 wRC+. This illustrates the larger difficulty using plate-discipline metrics to predict other offensive outcomes. Contact rates are really the only ones with a decent relationship to future outcomes, such as Isolated Power. None of the discipline metrics have a correlation to future wOBA above .30, except for Zone% (-.33), and that’s more about how pitchers approach the hitter given the overall talent of that hitter (i.e. better hitters tend to see fewer pitches in the zone). Even then, since 2007, no hitter besides Hamilton in 2012 has posted at least a 130 wRC+ when whiffing at more than 20% of pitches seen.
The jump in Hamilton’s whiff rate could simply be an isolated incident, with the rate set to drop a bit next year. But he’s now at an age where we generally see whiff rates increase for the rest of a player’s career. That’s not good, given that he’s be looking for substantial money — most of which will be tied to his offensive output. Unless he can regain some control over his O-Swing rate, it’s less likely we’ll see a favorable regression for his swings and misses.
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