2020 Starting Pitcher K% Increases

August 29, 2020

With the season nearly halfway finished, Nicklaus Gaut looks at the starting pitchers that have increased their strikeout-rate the most since 2019, looking at the factors that can tell us whether these are new skills here to stay, or just short-season randomness.

We're somehow halfway through the 2020 season and judgments need to be passed. Every statistic has a point where it is said to begin to stabilize and become more useful than it is not. For pitcher's strikeout-rates, that point is after they've face 70 batters. Note that it's "begin to stabilize", not "stabilize".  These statistics become more stable over time, with that stability increasing at a given rate. You'll know more about a pitcher after 100 batters faced than you will after 70 batters. But 70 batters faced is the point at which the correlation between that particular sample and one of a similar size has an r-squared of .49...AKA when the signal starts to outweigh the noise.

The fact that you can learn a lot about a pitcher's strikeout ability after so little time is certainly helpful but remember that it's just a starting point. For stabilization, but K% is also just a starting point for trying to determine what is behind the change. So, in addition to looking at the changes in K%, we'll look at changes in plate-discipline metrics like CSW% (called-strikes+whiffs/total pitches), SwStr% (whiffs/total pitches), Chase% (swings outside the zone), F-Strike% (strikes on first pitch+balls hit into play).

I'll never claim to be a mathematician but I do know that the above metrics are going to give you the best clues on the truth about a pitcher's strikeout ability (though, F-Strike% is in a class behind the others). For further reading on sample sizes in baseball statistics, I highly recommend reading this by Steve Slowinski or this by stabilization OG, Russell Carleton. And if you'd like to read more about the math behind which statistics correlate the best to K%, Chaz Steinberg wrote a comprehensive breakdown here. That goes beyond the scope of this article, however, because we just want to find out who's going to keep giving our fantasy team those sweet, sweet K's. Let's get to it.

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Biggest Increases In K%

Heading into the weekend, there were 45 starting pitchers who have increased their K-rate by at least one-point and have faced at least 70 batters. They are included below, along with their 2020 CSW% and the change in their CSW% versus 2019. To give a general sense of how their year has gone so far, as well as where they could be going, I've included ERA, FIP, and SIERA for added context.

In addition, available at the end of the article is a pop-out graphic that includes all of the below, with many of the additional plate-discipline metrics I've already mentioned, and color-coding according to the separate league averages.

Aaron Nola: 36.4% K% (9.5-point increase)

It turns out that the reports of Nola's demise were greatly exaggerated. Nola posted a 3.87 ERA in 2019 - after breaking out with a 2.37 ERA in 2018 - with ERA evaluators that didn't tell a different story (4.03 FIP, 3.82 xFIP, 4.14 SIERA). The Philly ace has roared back in 2020, putting up a 3.00 ERA through his first six starts, with a 3.41 FIP, 2.54 xFIP, and 2.91 SIERA. But the biggest change has been his new, fancy K-rate.

Even when Nola was at his best, his strikeout-rate stayed steady at around 27%. Until 2020:

What's Changed?

Nola's strikeout rate isn't the only thing that changed in 2020, his CSW (called strikes + whiffs) has also seen a dramatic jump, moving from an above-average 32.3% to an elite 37% CSW that trails only Shane Bieber among qualified pitchers. His called-strike rate stayed virtually the same, only moving from 20.5% to 21.2%, as the true owner of the jump belongs to his swinging-strike rate, which moved from 11.8% in 2019 to 14.8% in 2020.

While the whiffery on his curveball has risen slightly, going from a 16.5% SwStr% to 19%, the heavy lifting has been done by his changeup, which has a 21.6% SwStr% in 2020, up from 13.9% last season. In addition, Nola's changeup has a 72.2% GB%, and batters have only managed a .197 wOBA and .172 AVG against it, with a 0.0% barrel-rate.

The changeup has also moved to the forefront of his non-fastball pitch mix, after playing second fiddle to the curveball. The usage on the two pitches has nearly flipped since last season, with Nola now using the changeup 29% of the time, up from 19% in 2019. And the two-strike usage of his change has also risen significantly, increasing to 30% to RHB (up from 15%) and to 35% against LHB (up from 19%).

Here's Austin Riley casting his vote on whether Nola should throw the changeup more:

via GIPHY

Strikeout Sustainability

As long as Nola continues executing as he has been, there aren't many reasons to think the new strikeouts are going away. I don't know if Nola will carry a 33% K-rate over the course of a normal season. Maybe he'll top out at "only" 30% but the new pitch-mix (and the execution of said mix) points to a skills-change, not just a small-sample fluctuation.

Nola has a very manageable schedule the rest of the season. He'll face Washington next (the team he just beat after allowing two runs in seven innings and striking out eight), followed by a start against the Mets. After that, he'll likely have two starts in Miami because they're playing a seven-game series to makeup earlier postponements.

Zach Eflin: 33.3% K-rate (15-point increase)

Let's do another Phillie. Because you can add this to the list of things people say when giving examples of how crazy 2020 has been. Eflin's best strikeout-rate was 22.4% K% in 2018 and it dropped to an 18.3% K% last season. But through his first four starts, he has a strikeout-rate that, if qualified, would be the sixth-highest among starters. He's also raised his walk-rate up a few points and has a 5.12 ERA, so let's not get carried away.

What's Changed?

Eflin said during original spring training that he wanted to rely on the pitch that got him to the pros and get back to being a heavy sinkerballer. He's done just that, with the sinker going from 22.1% usage in 2019, to 56.6% so far this season, with his four-seam use dropping all the way down to 7%. His slider use has also changed dramatically compared to 2019. He's throwing it slightly less to right-handers (31% to 25%) but it's almost been abandoned versus left-handers (30% to 8%).

The velocity and movement profile on the sinker looks the same as before, but he's throwing it in the zone less (48.9% to 43.2%) and generating a lot more whiffs (5.9% SwStr% to 8.8% SwStr%). An inflated .515 woBACon against it seems bad but his .374 xwoBACon is right around league-average and the pitch also has a .458 BABIP thus far. That speaks more to the pitch getting a little unlucky, rather than pounded.

Check out Eflin throwing this wiffle-ball sinker past Giancarlo Stanton. Also, check out the early, world-class mustache he was rocking. If anyone is missing a big, fat catepillar, you might want to look above Eflin's upper lip.

via GIPHY

Strikeout Sustainability

Looking at the plate-discipline metrics above (along with the change since last season) I can believe some of the increased strikeouts in Eflin's game but 33% is a tough sled. A 12.1% SwStr% would support a rate in the 25% range, however, and his 63.3% contact-rate and 36% chase-rate are both elite. So I'm going to believe the new strikeout ability, just not at its current extremes.

Even with an increase in strikeouts, Eflin doesn't suddenly become a must-have fantasy commodity. But he might be worth giving a heavier weight when it comes to streaming. A 5.12 ERA is ugly but a 2.72 FIP speaks to a fair amount of luckiness. And his 3.21 xFIP and 3.42 SIERA say nice things when it comes to trying to estimate how Eflin's current skills might translate to more ratio success in the future.

Eflin is set to make his next two starts against Atlanta and Washington but the schedule gets wonky after that, with the Phillies making up for earlier postponements. They'll start with four games at the Mets and two games at home against Boston before playing the Marlins for seven straight games in Miami. After that, it's three more against the Mets and four versus the Blue Jays. And then the season will basically be over. A sub-4.00 ERA pitcher with a 25% K% would likely be useful for that stretch.

Yusei Kikuchi: 25% K% (9.5-point increase)

Much like Eflin, Kikuchi has had mostly terrible results, posting a 6.30 ERA through his first four starts, but looks much better according to his ERA evaluators, in addition to the strikeout-rate bump. The difference is that Eflin's rate went from mediocre to extraordinary, while Kikuchi's has gone from trash-heap terrible to above-average.

What's Changed?

First, there's the matter of his velocity bump. Kikuchi's velocity was down in 2019 compared to what it had been in Japan; he only threw seven pitches over 96 mph, hitting 97 mph just once. In 2020, he's already topped 96 mph a total of 22 times, with five pitches greater than 97 mph and has topped out at 98 mph.

He's now thrown 22% of his fastballs over 95 mph; that number was only 3.8% in 2019. Since arriving in the majors last season, batters have a .331 xwOBA against Kikuchi on pitches thrown between 92-94 mph; they have a .263 xwOBA on pitches over 95 mph.

Then there was a pitch mix overhaul. Gone is the curveball that he threw 15% of the time in 2019, while his slider usage has been nearly halved, going from 27% to 14%. His four-seam usage has dropped eight-points but the sea change is the addition of a cutter that has become his foundational offering so far in 2020, being thrown 43% of the time. In regards to his splits mix, left-handed hitters have seen cutters and four-seamers a combined 93%, with the slider thrown in occasionally. Its also a steady diet of the fastballs for right-handers, but they get more sliders (16%), as well as the occasional changeup (8%).

Here's the cutter in action (avert your eyes, Nick Mariano)

via GIPHY

At a glance, these mix changes seem to be positive. The curveball was tragic in 2019, allowing a .275 ISO and .409 wOBA against it. And the slider's being thrown less but has been much more effective after allowing a .342 wOBA against it in 2019. That's dropped to .225 wOBA this season, with the quality of contact against it also dropping significantly, going from a .406 woOBAcon to .249 (.350 xwOBAcon to .190). The slidepiece is getting fewer whiffs, dropping from a 14.7% SwStr% to 12.8%, but is generating way more groundballs, with its 71.4% GB% representing a 25-point increase from the year before.

Strikeout Sustainability

Taking a look at the most important metrics when it comes to judging strikeout ability, Kikuchi passes two with flying colors. His swinging-strike rate is up almost four-points and his contact-rate is down over 10-points. But his chase-rate has decreased slightly, while his first-strike percentage has seen a significant drop.

Going forward, I think the new K-rate is here to stay. Kikuchi was a strikeout pitcher in Japan and the numbers tell me he's much closer to that version of himself than he was last season. The cutter is working, the velo is poppin', and the slider's been much more effective, albeit no longer as his putaway pitch.

He has a tough matchup for his next turn, facing the young and hot San Diego Padres. But depending on what the Mariners want to do, in his next turn, he'll face either the Angels (.271 wOBA vs. LHB) or the Athletics (.329 wOBA). After that, he sets up to face the Rangers (.285 wOBA vs LHB) and the Diamondbacks (.264 wOBA).

* Update following Kikuchi's Thursday night start versus the Padres

Kikuchi picked up the win, allowing three runs and striking out six over five innings. The ERA was ugly (again) but he pitched pretty decently. He allowed a first-inning home run to Manny Machado (that'll happen) and the sequence on his second earned run went: infield single, double, infield single. Kikuchi's K% stayed virtually the same, moving from 25% to 25.7%.

The cutter was used 35% and had a 54% CSW, but more importantly, his four-seamer averaged 96.4 mph, and he went over 97 mph seven times. The velo is poppin' and the new cutter is doing some work. He's going to give up some home runs but the new K-rate is for real.

Other Leaderboard Takeaways

Shane Bieber: 42.9% K-rate (12.7-point increase)

What can I say about Shane Bieber in 2020 that hasn't already been said about the Sistine Chapel, a perfect sunrise, or the Fast & Furious film anthology? At some point, you just run out of adjectives.

Bieber told everyone in his first start of the season what was going to be up in 2020, striking out 14 Royals in six innings, but it's taken the collective awhile to come all the way around. He has some of the best command in the majors and has multiple pitches that make batters look silly. His 20.7% SwStr% is the tops among qualified starters, just as his 54.5% contact-rate is the lowest. He really only lags behind with his chase-rate, which is all the way down at the third-highest. He makes up for it, though, by having the league's highest CSW%.

Trevor Bauer: 39.8% K-rate (12-point increase)

From a current Indian to a former one, Bauer has really taken off since deciding to no longer throw the ball over the centerfield fence. Besides the drop in contact-rate, the changes in his other plate-discipline metrics don't exactly send a shiver down my leg. Especially the near four-point drop in his chase-rate, which was already mediocre.

What's amusing is that after years of telling everyone who would listen how easy it is to jack up your spin-rates through nefarious means, Bauer has (coincidentally) jacked his spin rates up to otherworldly levels.

Here are his 2020 spin-rates, along with where each pitch ranks. Also included are his RPM increases, along with his rank according to the amount of spin changed:

Maybe Bauer has discovered a new and magical way to gain tons of spin in a manner that our simpleton brains just can't comprehend. Or, maybe he's just doing what he's been threatening to. We'll have to see how far it takes him.

Zach Plesac: 31.2% K-rate (12.7-point increase)

I'd love to talk about the big steps that Plesac had seemingly taken this year, posting a 1.29 ERA (2.39 FIP, 2.97 SIERA) to go along with an elite strikeout-rate. We could talk about how his slider/curveball usage has jumped up, along with their swinging-strike rates, with the slider going from 17.2% to 24.1%, and the curveball going from 6.8% to 15%. I might have even mentioned that the improved whiffery on the curve may be due to a significant change in its movement profile, with the hook being considerably tightened up on both planes.

We could've been talking about those things but unfortunately, Plesac was a selfish idiot who broke quarantine and lied to his teammates about it, even though one of them has recently recovered from cancer. So, maybe next year.

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