Dallas Keuchel and Pitcher Plate-Discipline Aging Curves

The first time Dallas Keuchel broke out, it was because he found a new pitch. Moving from the curve to the slider really pulled his arsenal together. The second time he broke out, it was for reasons that were both more complicated but also just as conventional — it looks like Keuchel merely threw fewer pitches inside the zone, while getting batters to reach and swing just as much. More swings on pitches outside of the zone means more misses and more strikeouts.

All of that is nice, but it’s hard to know which breakout is easier to believe. Our intuition probably tells us that the first is less delicate — he needed a breaking ball, and he found a good one, and it should remain good. But batters could adjust more easily to the second one, couldn’t they? Just lay off more of those balls?

In the context of the PITCHf/x era, Keuchel’s drop in zone percentage was extreme but not an outlier. Baseball Info Solutions had him dropping 2.5 percentage points in that category, which would have ranked him among the most extreme 27% of drops since 2002 (minimum 120 innings pitched in both seasons). PITCHf/x has the drop as 3.8 points, which would put his 2015 season in the top 6% since 2008.

Did other pitchers that dropped their zone percentage as much retain that change in zone percentage the next year? On the BIS side, pitchers that dropped as much as Keuchel or more saw their zone percentage rise 0.7 points the year after their four-point drop. Roughly 57% of them threw the ball in the zone more often the following season. By PITCHf/x, starters as extreme as Keuchel saw their zone percentage increase 1.1 points the next year, and 62% of the pitchers threw the ball in the zone more often.

So we have a more than 50% chance that Keuchel throws the ball in the zone more often next year, and ostensibly gets fewer swinging strikes as a result. How does this fit into our concept of how pitchers age?

We know that swinging strike rate is more stable for starters rather than relievers, suggesting that size of arsenal is part of the picture. But in this case, Keuchel’s second breakout was a result of fiddling with his approach to the zone. How do those zone stats age? Thanks to Jeff Zimmerman, using the delta method, we have aging curves for pitcher plate-discipline stats.


That yellow line leaps out at you, especially in Keuchel’s situation. Pitchers throw the ball in the zone less year after year, it looks like. And since we know that contact rates are worse on pitches outside of the zone, we have more of an idea of how starters stave off aging better relievers: they throw fewer fastballs, and they throw them in the zone less often. That’s a bit of conventional wisdom coming true in the numbers.

What does this mean for Keuchel in particular, though? It at least suggests that this second breakout came through means as conventional as the first. Pitchers get better by adding pitches, and then by throwing those pitches more often and outside of the zone more often. Or, at least, that’s how pitchers are able to produce roughly the same numbers even as time robs their athleticism (read: velocity). Keuchel has now done both of those things in consecutive years. He’s just reading the playbook. Everything is fine.

Except, you have to wonder about how extreme this second move was. Yes, pitchers generally nibble more as they age, but Keuchel, by one measure, did this harder than 90% of the league. Is he pushing the Old Pitcher buttons too hard?

Let’s return to the PITCHf/x list, since it’s shorter. Let’s look at only pitchers more extreme than Keuchel in the year they dropped the zone percentage, and then their follow-up year. You’ll notice that the list is, by and large, populated by pitchers older than Keuchel. (Pitchers ordered by largest decrease in zone percentage.)

Starting Pitcher Extreme Zone Percentage Droppers
Name Start Season Start Age pfxZone% yr1 pfxZone% yr2 pfxZone% yr3 YR2 FIP Yr3 FIP Diff FIP
Wan. Rodriguez 2009 30 53.6% 46.9% 46.6% 3.50 4.15 0.65
James Shields 2011 29 51.4% 45.0% 47.7% 3.47 3.47 0.00
Kevin Millwood 2008 33 54.7% 48.7% 50.0% 4.80 4.86 0.06
Rick Porcello 2010 21 54.3% 48.7% 48.4% 4.06 3.91 -0.15
Jer. Hellickson 2011 24 49.1% 43.5% 45.9% 4.60 4.22 -0.38
Edwin Jackson 2011 27 50.1% 44.7% 46.5% 3.85 3.79 -0.06
Aaron Cook 2008 29 52.9% 48.2% 51.3% 4.60 4.54 -0.06
Zack Greinke 2008 24 52.7% 48.1% 49.0% 2.33 3.34 1.01
Derek Lowe 2009 36 42.9% 38.3% 37.1% 3.89 3.70 -0.19
John Lackey 2008 29 54.6% 50.1% 51.5% 3.85 4.71 0.86
Ricky Nolasco 2008 25 56.7% 52.3% 52.8% 3.35 3.86 0.51
Fran. Liriano 2010 26 48.0% 43.6% 43.2% 4.54 3.54 -1.00
Johnny Cueto 2010 24 49.1% 44.8% 47.4% 3.45 3.27 -0.18
Doug Fister 2010 26 57.7% 53.5% 51.8% 3.02 3.42 0.40
Jake Peavy 2011 30 53.5% 49.4% 51.4% 3.73 3.96 0.23
His. Iwakuma 2013 32 51.4% 47.4% 46.8% 3.25 3.74 0.49
Jeff Francis 2010 29 52.6% 48.6% 46.6% 4.10 4.27 0.17
Shaun Marcum 2010 28 48.8% 44.8% 46.0% 3.73 4.10 0.37
A.J. Burnett 2010 33 48.6% 44.7% 51.2% 4.77 3.52 -1.25
Jake Peavy 2013 32 51.4% 47.5% 50.6% 4.11 3.87 -0.24
Kevin Correia 2009 28 53.8% 49.9% 51.7% 4.71 4.85 0.14
Just. Masterson 2009 24 55.1% 51.2% 55.4% 3.93 3.28 -0.65
Jason Vargas 2011 28 53.5% 49.7% 47.9% 4.69 4.09 -0.60
Freddy Garcia 2010 33 50.2% 46.4% 45.0% 4.12 4.68 0.56
Cole Hamels 2011 27 52.3% 48.5% 50.3% 3.30 3.26 -0.04
Sample Average 28.3 52.0% 47.4% 48.5% 3.91 3.94 0.03
Dallas Keuchel 2014 26 44.3% 40.1% 2.91
Minimum 120 innings thrown in all three seasons, overall n = 442

If we cut the list to just the 10 pitchers who were 27 or younger — Keuchel was 26 when he started this process in 2014 — things change. Those young pitchers actually cut five points off their FIP the next year, ostensibly because they occupied a different place on the aging curve. If you cut the list further to include only excellent young pitchers the year after they cut their zone percentage, you’re talking about Zack Greinke in 2013 and Doug Fister in 2012, and good luck figuring out which one Dallas Keuchel is going to be.

But now we’re talking about predicting Keuchel’s next season off of a sample of 10 (or, even worse, two). And you might also notice that Keuchel is even more extreme than these fellow extreme droppers, since he dropped to nearly 40.1% pitches in the zone. He wasn’t around the zone anywhere league average before (47.8% was league average last season) and now he’s hitting the zone less often than every qualified pitcher not named Kyle Gibson or Francisco Liriano.

So Keuchel is not likely to throw fewer pitches out of the zone next year, he’s already fringe in that regard. Every pitcher throws fewer pitches in the zone as they age, though, so it’s fair to ask if he pushed this button too hard, to quickly. He’s just had the sixth-lowest PITCHf/x zone% of the era, and so maybe he can’t keep dropping that percentage much further. Maybe it has some sort of implications for his longevity, since he’s starting at such a low point.

But when it comes to next year, there isn’t much to suggest that he can’t do the same thing again. After all, it’s the Aging Pitcher Playbook — add pitches, throw more junk, and nibble more. It’s just weird to see that playbook used by a pitcher in his prime to become more excellent. Weird enough that we don’t know exactly how sustainable it is.

With a phone full of pictures of pitchers' fingers, strange beers, and his two toddler sons, Eno Sarris can be found at the ballpark or a brewery most days. Read him here, writing about the A's or Giants at The Athletic, or about beer at October. Follow him on Twitter @enosarris if you can handle the sandwiches and inanity.

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Green Mountain Boy
6 years ago

As I recall, Tommy John and Tom Glavine lived outside the zone for the better part of their careers and batters just kept chasing and chasing. No reason to expect the same strategy won’t continue working for Keuchel.