Prognosticating Punch Outs and Passes, Free
One month of baseball is in the book; enough for walks and strikeouts to become 50% reliable for pitchers, or at least 50% reliable indicator of past talent. As Eno Sarris highlighted in his recent starting pitcher rankings update, velocity, and strikeout minus walk rate are two indicators fantasy baseballers should be primarily focused on this early into the season. Strikeout minus walk rate is perhaps the best in-season predictor of future performance. It’s still early but April performance offers some limited insight on the rest of the season.
TLDR: this article examines expected strikeout and walk rates for the rest of the season based on April pitch results (swinging strikes, foul strikes, looking strikes, balls). Scroll to the bottom for expected strikeout rate minus walk rate leaderboards (updated through 5/3 games).
With inspiration from Mike Podhorzer’s expected strikeout rate and expected walk rate equations, I built very similar models using April 2018 performance (using pitchers with minimum 380 pitches) to forecast post-April 2018 performance. My models had virtually identical predictive validity to Podhorzer’s models on the 2018 sample. My models take into account swinging strikes, foul strikes, looking strikes, and balls to predict strikeout rate and walk rate—all gathered from Baseball Savant. My expected strikeout and walk rate models each explained about 50% of post-April 2018 pitcher performance. It is reasonable to conjecture they might also explain about 50% of post-April 2019 performance. That being said, at least 50% of pitcher performance remains unexplained by these models. One should take performance in prior years very seriously when setting expectations for the rest of this season.
The table below shows the values needed to calculate expected walks and strikeouts.
Interested readers can pull data from Baseball Savant themselves and use these equations to generate expected strikeout and walk rates. Just multiply each rate by the appropriate coefficient from the table and then add the intercept. For example, to calculate expected swinging strike rate for Caleb Smith, first multiply his current swinging strike rate, 18.3%, by .016. Here’s the full calculation for Caleb Smith: -.4396+(18.3*.0161)+(15.7*.0103)+(19.5*.0079)+(32*.0045) = 31.5% expected strikeout rate.
Swinging strikes captures three types of swinging strikes, foul tips, and missed bunts. It’s close to Fangraphs’ swinging strikes, but about 1% higher because it captures foul tips (league average 12% of all pitches for my measure versus league average 11% for Fangraphs’ measure). Foul strikes captures fouls, foul bunts, and foul pitchouts (17% league average). Called strikes captures called strikes (17% league average). Balls captures balls in dirt, balls, and hit by pitches (not intentional balls) (37% league average).
This article concludes with a rankings leaderboard of all pitchers by expected strikeout rate minus expected walk rate (minimum 380 pitches in 2019, or about four starts, updated through 5/3 performances). kwERA is an ERA estimator from GuyM, explained by Tom Tango here, that only uses strikeout and walk rate. I set league average kwERA to 4.15 for my sample. League average expected strikeout rate is 23.2%. League average expected walk rate is 7.7%. League average strikeout minus walk rate is 15.5%.
I have included 2018 data in the leaderboards so it’s easy to see how pitchers have changed since last season (not that anything is particularly easy to see in this admittedly very busy table!). There’s a lot of information and I wanted to give readers a chance to draw their own conclusions. I also share some of my own observations below.
- Expected strikeout and walk rates underrate certain guys who have been ostensibly “over-performing” since last season. One example is Hyun-Jin Ryu’s expected walk rate of about 7%. His walk rate has been sub-5% since the beginning of 2018, for over 100 innings now, so I’d expect him to continue to outperform his expected walk rate. Another example is Trevor Bauer’s 27% expected strikeout rate. He’s been above 29% since the start of 2018, and his 2019 components are pretty similar to last year. I’d be hesitant to forecast too much regression in his strikeout rate.
- A few surprising and spectacular early season performances are supported by expected strikeout minus walk rates: Caleb Smith, Matthew Boyd, and Chris Paddack. Boyd and Smith are pitching better than they have earlier in their careers and some regression should be expected, especially since their pitch mix and velocity remains similar to pre-2019 (though Smith is throwing a few fewer fastballs and a few more change-ups). Even with regression, though, I’d expect them to remain top 35 starters in 2019–they’ve simply been too fantastic thus far to reasonably expect much less. Paddack is riding a sky-high foul rate and minuscule ball rate to stellar results. Given his generational minor league performance — I’ve previously argued it was among the best in the past 20 years — as well as the ascendant first month and developing curveball, he looks very much like a budding ace. Note his best start of the year against the Mets was not included here, and he’d look even better accounting for it.
- Some people are panicking about veteran stalwarts off to slow starts, including Chris Sale, Noah Syndergaard, Carlos Carrasco, and Jacob Degrom–though each has alleviated fears somewhat with strong recent starts. Expected strikeout and walk rates suggest one shouldn’t worry too much about them–they each rank in the top 15 in the league by this measure.
- Sale’s velocity hasn’t been this low since 2016 (it was also in the 93-94 mph range in 2016) when his ERA and SIERA were both in the 3.30-3.40 range rather than the sub-3 range we’ve come to expect from him. His swinging strike rate is also down considerably from last year. It’s probably wise to expect closer to 2016 Sale rather than 2018 Sale–but 2016 Sale was still a top pitcher. Carrasco is perhaps the best (and somewhat obvious) buy-low target given his awful start. With similar velocity, pitch mix, and spin rates to 2018, he is an easy bet to bounce back.
- I would not soon target Dylan Bundy, Corbin Burnes, or Freddy Peralta in most leagues despite relatively strong expected strikeout minus walk rates. Each of these guys is flyball and homer-prone, a trait missed when only considering strikeouts and walks. I do think each of them has been better than their performances indicate. I’d keep a close watch on Bundy if he gets traded out of Baltimore. I also expect Peralta will be demoted to the bullpen or minor leagues before receiving another start, but I’d keep an eye on him to see if he figures things out (update: he pitched five strong innings out of the pen last night). He was promising in the majors in 2018 and has a strong minor league track record.
- Aaron Nola and Walker Buehler have had pretty mediocre starts this year (though their fantasy owners may choose a stronger adjective) after very strong 2018 performances. Each has too good a pre-2019 track record to justify panicking yet, and neither has lost velocity. I’d be comfortable betting on both to bounce back despite weak expected and actual performances thus far — and they’ve both already begun to improve in recent starts. It is worth noting Nola was pretty lucky in 2018, and a mid-3s ERA true talent pitcher is a more reasonable expectation than the sub-3 ERA guy he was in 2018.
- I’m pretty worried about Brad Peacock’s ability to stick in the rotation with Joshua James and Forrest Whitley ready to take over. In any case, he’s probably an improvement over the enigmatic Wade Miley, who has somehow managed a sub-3.30 ERA since 2018 despite terrible strikeout rates.
- J.A. Happ and Masahiro Tanaka are a bit more worrying than the usual slow starters given their advanced ages. Happ has also lost around two mph of velocity, while Tanaka has lost a half mile per hour. I’m more worried about Happ, but both of these slow starts are worth monitoring closely.
- Patrick Corbin is another guy I’m slightly worried about. His swinging strikes have declined almost five percent this year compared to his career year in 2018–the single biggest year-over-year decline in the sample. Some of the decline might come from decreased slider velocity of almost one mile per hour early on compared to 2018. His most recent start versus the Phillies, however, was promising in this regard: he posted his third highest single-game average velocities since the start of 2018.
Finally, here are the full expected strikeout minus walk rate leaderboards for 2019, sorted by kwERA, where 4.15 is league average.