Why has Danny Duffy’s strikeout rate dropped off? Breaking down what's changed

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Thursday May 18th, 2017

Pull up the league trailers in strikeout rate, and you’ll see a host of familiar names. Bronson Arroyo. Jeremy Hellickson. Jered Weaver. Bartolo Colon. No one should be surprised they’re among the pitchers who have struck out the lowest percentage of batters they’ve faced this season. Those four are simply playing to script.

There’s one name just barely ahead of them on the strikeout rate leaderboard, however, that is a shock. If there’s one pitcher who is not playing to script this season, at least as far as strikeouts are concerned, it’s Danny Duffy. The Royals lefty ranks 78th in the league with a 16.7% strikeout rate. This comes on the heels of a season in which he fanned 188 batters in 179 2/3 innings and amassed a strikeout rate of 25.7%, which ranked 12th in the majors, ahead of Chris Sale, David Price and Jake Arrieta. Remember that last name. We’ll be revisiting it shortly.

Duffy made a star turn last season that was seemingly built to last. Look, I even wrote about it. Duffy always had the stuff, but was never able to turn that natural ability into consistent top-of-the-rotation success. After dominating in the bullpen early last year, Duffy brought a reliever’s look to the starting rotation. Specifically, he abandoned the windup, pitching solely out of the stretch. That eliminated all the moving parts in his rotation and helped him find a repeatable delivery. The results he produced after that change spoke for themselves.

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That last sentence still applies to Duffy this year. The results are speaking for themselves. They’re just telling a much different story. Duffy is fine in the ERA department, where his 3.38 mark is solid and matches up nicely with his 3.34 FIP. He owns a 1.34 WHIP, however, and an 8.1% walk rate to go along with his anemic strikeout rate. Last year, the spread between Duffy’s strikeout and walk rates was 19.9%, good for eighth in the majors. This year, it’s 8.6%, which places him 73rd among 99 qualified pitchers. No pitcher is going to get the job done with that slim a difference between strikeout rate and walk rate, least of all a pitcher who is supposed to live on missing bats.

Here’s the thing, though. Duffy is still missing plenty of bats this year. His whiff rate is 12%, essentially right in line with last year’s 12.9%. Duffy is one of 21 pitchers in the majors this year with a whiff rate of 11.3% or better. Of those 21, Duffy is one of two with a strikeout rate lower than 22.7%. The other is Masahiro Tanaka. Remember, too, that Duffy isn’t just below that threshold. He trails it by a full six percentage points. Including Tanaka, the other 20 pitchers with a whiff rate of 11.3% or better have an average strikeout rate of 28.3%, and that sweeps in four pitchers whose whiff rates are lower than Duffy’s. Clearly, the simple goal of getting whiffs isn’t Duffy’s issue.

Here’s where Arrieta re-enters our picture. Arrieta had a season for the ages in 2015, posting a 1.77 ERA, 0.86 WHIP and 236 strikeouts in 229 innings, en route to the NL Cy Young Award. That season, Arrieta notched a 27.1% strikeout rate and 5.5% walk rate, which are awfully close to Duffy’s 25.7% and 5.8% rates last season. They’re certainly close enough to serve as the basis of an apples-to-apples comparison.

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Arrieta remained among the best pitchers in 2016, but he didn’t come close to matching what he did the previous season. Specifically, his strikeout rate dipped while his walk rate skyrocketed. Arrieta struck out 23.9% of the batters he faced last season while walking 9.6% of them. Those dramatic shifts in results suggest he was an entirely different pitcher, but that couldn’t be further from the truth. From velocity, to pitch usage rates, to command and control, Arrieta was statistically identical in 2016 and 2015. It wasn’t Arrieta who changed, but the approach of the hitters who faced him.

O-swing rate measures the frequency with which hitters swing at pitches outside the strike zone. Often, a pitcher just misses the zone with a pitch. However, all pitchers throw chase pitches, especially those who strike out a lot of batters. O-swing rate does a great job of measuring how many swings they’re inducing on those pitches. In 2015, Arrieta racked up an o-swing rate of 34.2%. Last year, it fell to 29.6%. That, more than anything else, was responsible for Arrieta’s changing fortunes with respect to strikeout and walk rates. The exact same issue is befalling Duffy this season.

When Duffy was making his way to a 25.7% strikeout rate and 5.8% walk rate last year, he was getting hitters to swing at 32.1% of pitches out of the zone. This year, they’re doing a much better job of laying off those pitches, pushing Duffy’s o-swing rate down to 27.8%. That’s a drop of 4.3 percentage points, which, for all intents and purposes, is identical to Arrieta’s 4.6-percentage-point slide from 2015 to 2016. In short, Duffy, like Arrieta before him, isn’t getting as many swings on pitcher’s pitches. That’s showing up directly in his strikeout and walk numbers.

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Before going any further, we must point out the velocity problem. Duffy’s average fastball velocity has fallen to 93 mph after sitting at 95 mph a season ago. That clearly factors into the dip in strikeout rate, but it doesn’t explain his declining o-swing rate, or the increase in walk rate. Still, it’s something we have to keep in the back of our minds.

Duffy may be producing whiffs at the same rate as he did last year, but he’s not getting them at the same time. The best way to illustrate this is by looking at two-strike counts. Last season, Duffy amassed a 64.8% swing rate and 19.4% whiff rate with two strikes. This season, those rates are down to 61.1% and 13.6%, respectively. Eighty-one of Duffy’s 221 two-strike pitches, 36.7% of them, have been taken for balls. Last year, Duffy registered a ball on 30.5% of his two-strike offerings.

There is, of course, the chance that Duffy is simply throwing worse pitches this year. That, last season, his chase pitches were closer to the zone or set up with better sequencing. Thanks to Statcast, we can rule out at least one of those potential explanations.

First, let’s consider the idea that Duffy’s balls on two-strike counts are farther out of the zone this year. These side-by-side zone profiles and heat maps should dispel that notion. In both comparisons, the left is 2016 and the right is 2017.

Let’s start with the zone profiles. Duffy’s hot zones are the same this year as they were last year. When he goes outside the zone with two strikes, he still misses just off the plate east or west, but in the middle of the zone north or south, or just below the zone. As expected, the zone in which he misses most frequently is down and in to a righty, down and away from a lefty, which is where his slider will travel naturally.

As for the heatmaps, this year’s smaller sample is helping create two hot zones, but generally there’s no difference between 2016 and 2017. He’s still peppering his arm-side of the zone with two-strikes. As the season wears on and Duffy racks up more pitches, we can safely expect this season’s heat map to resemble last season’s.

On the flip side, there might be something to the idea that Duffy’s sequencing this season is less effective. First, the charts. Here are Duffy’s pitch-usage rates in two strike counts over the last two seasons, with 2016 first.

That’s an awfully precipitous fall for Duffy’s four-seamer in two-strike counts. It turns out the two-strike four-seamer was quite effective for Duffy last season. It had a 16.9% swinging-strike rate and was responsible for more than one-third of his two-strike whiffs. This year, he has produced just two whiffs with the four-seamer in two-strike counts. This brings us back to Duffy’s velocity, and it certainly is part of his strikeout problem this year. But, again, it doesn’t help with respect to his plummeting o-swing rate.

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We’re left with the same explanation that Arrieta had to deal with last year. Hitters have changed their approach, and are now waiting him out with the confidence that he’s going to try to expand the zone. Last year, Duffy averaged 3.7 pitches per plate appearance. This year, that’s up to 3.76. While that may not seem like a significant difference, it’s another piece of evidence that hitters are working longer counts against Duffy this season. Given the fall in o-swing rate and the fact that his zone profile hasn’t changed in a meaningful way, we can infer that most of those extra pitches are the result of them laying off pitches they swung at a season ago.

Now it’s up to Duffy to adjust. We’ve already seen him make one huge adjustment in his career, ditching the windup for the stretch in all situations, and how well it worked for him. It’s possible he has lost some confidence attacking the zone because of his decreased velocity, but it’s clear that he can’t expect hitters to flail at the same pitches out of the zone this year that they did last year.

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