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With all of the data at our fingertips, we can really do some interesting things in the offseason. We know that the strikeout rate is one of (if not the) most important metrics for a pitcher. It's hard to have success with a low strikeout rate, and it's a bit tough to not have success with a high strikeout rate.
So, to get ahead of the crowd, we want to have a good feel for pitcher strikeout rates ahead of the 2023 season. We can get a lot of the way there just by looking at strikeout rates from 2021-2022 (these are pretty stable year-over-year), but any old fart can do that! I want to take the next step and look at some underlying metrics and see if we can make some predictions on pitchers likely to increase their strikeout rate moving forward.
We will first test some different metrics and see which ones correlate most strongly with the strikeout rate, and then we'll look up some pitchers that stand out in those categories once we find them.
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Predicting Pitcher Strikeout Rates - Correlation Check
One quick way to get an idea of which stats predict other stats is to check their correlations. I used all pitcher seasons from 2019 through 2022 where a pitcher faced at least 500 batters (that's about 20 starts), and got the stats below on them for each season.
- K% (strikeout rate, the percent of all batters faced that they struck out)
- SwStr% (swinging strike rate, the percent of pitches the pitcher threw that were swung and missed at)
- Whiff% (whiff rate, the percent of swings that were whiffed on)
- CSW% (called strike plus swinging strike rate, it's swinging strike rate but it includes called strikes as well)
- Primary Fastball SwStr% (the swinging strike rate the pitcher put up on his most commonly thrown fastball)
- Primary Fastball Velocity (self-explanatory)
- Strikeout Pitch SwStr% (I found the pitch that the pitcher most commonly threw on 0-2, 1-2, and 2-2 counts and got the SwStr%)
- First Pitch Strike (the percent of 0-0 pitches that were in the strike zone)
This gave me a sample of 312 pitcher seasons, here's a quick peek at the data just to get a feel for it:
If you want the full data, you can have it!
Pitching Stats Correlation Matrix
Now to the moment of truth, the correlation matrix! Remember:
- The closer the number gets to +1, the stronger the positive correlation is (as one rises, so will the other)
- The closer the number gets to -1, the stronger the negative correlation is (as one rises, the other will drop, and vice versa)
- If the number is between -0.5 and 0.5, there is a very weak or non-existent relationship (the closer to zero, the weaker it becomes)
What We Find - Which Stats Predict Strikeout Rates
1. Each stat is perfectly correlated with itself, that's obvious - so just ignore that diagonal of 1's.
2. The winner is actually whiff% with a very strong 0.9 correlation coefficient. This narrowly edges out SwStr% at 0.88. That's a bit surprising, but the numbers are so close that it may as well be considered a tie. I have always preferred looking at SwStr%, and I have left Whiff% completely out of my dashboards and whatnot, so maybe it's time to bring that metric more into the light!
3. CSW% is a close third at 0.85. You might think that CSW% would be more predictive since it fully encompasses SwStr% and then adds more data to it, but it turns out that called strikes are noisy/random enough to actually make things worse. It's better to stick to SwStr% when predicting strikeouts - although CSW% is a better metric for also predicting walks.
4. After those three, we get into the weak correlation range. The SwStr% on the primary fastball comes in at 0.61 - so there's some correlation there, but a pretty weak one. That's a little surprising considering the primary fastball typically takes up 35-50% of a pitcher's arsenal. If we compare primary fastball SwStr% to overall SwStr% - it's another pretty weak correlation at 0.62. This tells us that a pitcher's SwStr% on his primary fastball has very little to do with the SwStr% on his other pitches.
5. Strikeout pitch SwStr% doesn't correlate. This isn't surprising to see after we saw the mark on the primary fastball. The strikeout pitch is a minority of the pitcher's arsenal, so it's not surprising that it doesn't predict the rest of his pitch performance.
6. Velocity basically doesn't matter.
7. First-pitch strike rate doesn't matter at all. It may correlate a little bit with BB%, but it has nothing to say about a pitcher's K%.
We can visualize these relationships with scatter plots, which doesn't add a lot of value here, but it's fun for me so I'm going to do it! Here is our strongest relationship visualized, K% vs. Whiff%:
You could pretty much draw a straight line going upwards from left to right through those dots, which is a sign of a strong positive relationship. As your Whiff% goes up, so will your K%.
Top whiff rates of the last four seasons, along with their respective strikeout rates:
- Corbin Burnes (2021): 37.3% Whiff%, 35.6% K%
- Gerrit Cole (2019): 37.2% Whiff%, 39.9% K%
- Luis Castillo (2019): 35.8% Whiff%, 29.0% K%
- Corbin Burnes (2022): 35.2% Whiff%, 30.5% K%
- Spencer Strider (2022): 34.9% Whiff%, 38.3% K%
If we look at all pitchers that went above a 30% Whiff%, the lowest K% we can find is Luis Garcia's 26.4% mark in 2021 - still a pretty strong strikeout rate!
Our weakest relationship was the first-pitch strike rate vs. K%.
The top pitchers in this stat:
- Chris Paddack (2019) : 63.7%, 27.0% K%
- Julio Urias (2021): 63.6%, 26.3% K%
- George Kirby (2022): 61.3%, 24.5% K%
- Yu Darvish (2022): 61.2%, 25.6% K%
- Rick Porcello (2019): 61.1%, 18.7% K%
Corbin Burnes has posted well below-average first-pitch strike rates in his career, and he's been one of the best strikeout pitchers in recent memory. Pitchers with this kind of stuff just don't really have to worry about throwing the first pitch in the zone, because they have so much ability to get whiffs and to catch up from behind in the count. Don't concern yourself with this stat.
Which Pitchers Stand Out for 2023 Increases
So we come to a pretty simple conclusion. We want to focus in on SwStr% and Whiff%, and see which pitchers excelled in these categories without excelling in K%. This gives me a great opportunity for another scatter plot!
I have focused on 2022 data and loosened the criteria to just 250 batters faced, about 10 starts worth (this does put some relievers into the data, but oh well). Then we plot each pitcher's K% alongside their Whiff% and see who falls furthest away from the trend line.
Link to full interactive view of the plot.
Since these two stats are so highly correlated, there aren't going to be a ton of outliers. Alex Lange (reliever for the Tigers) is the only dot that really sticks out, but he's not even a starting pitcher so we aren't too worried about that.
I checked out the numbers and picked out some notable starters to talk about. Keep in mind that the league average whiff rate in 2022 was 25.5%, and the league average strikeout rate in 2022 was 22.5%.
#1 Brayan Bello, Boston Red Sox
He went for a 26% Whiff%, but just a 20.6% K%. That's above average in Whiff% but significantly below average in K%. I imagine it's a bit more common to see this kind of thing happen with rookies. Bello came up in the middle of the year and looked completely unfit for a job in a Major League rotation at first, but improved on his second try and finished the season on a pretty nice run of starts.
He went for a 22.1% K% over his last five starts, and his changeup was dominant with a 22% SwStr%. There is certainly some work to be done, but he did show the ability to generate whiffs in the Major Leagues right away - which is a great sign for the future. I expect a significant strikeout rate improvement in 2023.
#2 Roansy Contreras, Pittsburgh Pirates
The Buccos have a knack for not getting much out of very talented pitchers, but it's too early to say that Contreras is doomed in Pittsburgh. His whiff rate in 2023 was a very strong 27.7%, but the K% did not follow at just 21.1%. His SwStr% was also strong at 13.2%. It was really surprising how few strikeouts he got. Looking at his pitch arsenal, he's basically a two-pitch guy, but both pitches have promise.
He already has an elite slider which racked up a sick 23.4% SwStr% and a .237 xwOBA against in 2023. The problem was his four-seamer, which went for just an 8.6% SwStr% and got crushed by hitters to the tune of a .404 xwOBA. The pitch has plenty of velocity on it though, so he at least seems to have the arm talent to improve on that pitch. I find it really hard to believe Contreras is a below-average strikeout pitcher again, especially considering the 33% K% he put in AAA last year.
#3 Edward Cabrera, Miami Marlins
He had the 24th-best whiff rate in this data sample at 31.3%, but his K% was much lower at 25.8%. He is another very young starter at the age of 24, and he struck everybody out in the minors to the tune of a 36% strikeout rate. He leads his arsenal with a changeup (33%) and a curveball (21%) before getting to his fastball (20%), so that's an uncommon pitch mix. There are certainly more questions to be answered with Cabrera given this funky pitch mix and his problems with the walk (11% last year), but the strikeout upside is certainly here.
#4 Jordan Montgomery, St. Louis Cardinals
He was a guy I was on last summer as someone excelling in SwStr% but falling short in K%. At the end of the year, that remained true for the new Cardinals lefty. He posted the 83rd-best whiff rate in this sample (27.5%), but only the 147th-best strikeout rate (21.8%).
Unlike the other names here, Montgomery is not a young guy (he's 29), so it's a little bit less believable that he can pile on 3-5 points on the strikeout rate given that his career mark (22.8%) isn't far off from what we saw in 2022. Still, the southpaw clearly has the ability to get whiffs - and that just might turn into a 25%+ K% one of these years, especially now that he's escaped the tough AL East division.
#5 Patrick Sandoval, Los Angeles Angels
Sandoval went for the 48th-best whiff rate (29.6%) in the sample but finished just 109th in strikeout rate (23.7%). He also gets it done with a changeup (a sick 24.2% SwStr% on that pitch), but in 2022, he also improved his slider (19.2% SwStr%). Those are his two main pitches, so this is another uncommon arsenal. His four-seamer is bad with a 5.7% SwStr%, and he also features a sinker with an SwStr% of 5.4%.
This might be a common thread here. Pitchers throwing a ton of breaking stuff can get whiffs but have a tougher time getting strikeouts without the established fastball. Hitters are a bit less likely to chase on 0-2 or 1-2 when they have already seen a few of those changeups and sliders that rarely end up in the strike zone, but I am pretty much guessing at this point. Sandoval is one of the game's better pitchers at generating whiffs, and I would not be surprised to see him post a really solid K% at some point in the near future.
There are five names to ponder, but there is plenty more to explore in the plot and data I've provided - happy searching!
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