Studying Strikeout Rates: Whose Will Rise & Whose Will Fall?

Masahiro Tanaka has carried the Yankee staff this season thanks to his league-leading Swinging Strike Rate.

When looking at early season statistics, it’s important to see what can be sustained and what cannot. A third of the season is already in the books, and when it comes to projecting the final two thirds of the year only one thing is certain: some players will regress from unrealistic paces, and others will improve from slow starts to the season.

One way to see what pitchers have under or over-performed is in their strikeout rate. Theoretically, the correlation between the amount of swing-and-misses a pitcher gets and the amount of strikeouts they get should be nearly 1 –  however, strike outs looking, “clutch” swing-and-miss pitches on strike 3, and pure variance will lead to a much lower correlation. I took a look at this year’s 102 qualified pitchers and compared their K% rate and their SwStr% rate:

I created the “expected K%”, or xK%, for each pitcher by normalizing their swinging strike rate to the yellow line on the graph, which shows the best-fit line of the data for all 102 qualified pitchers. This is what in the long-run will be the average K rate of a pitcher with that given swinging strike rate. While some pitchers may be better at getting strikeouts looking or reaching back for a swing-and-miss on strike 3, this is where most pitchers should end up. The correlation so far this year is at .55, while last year at the end of the year the correlation was .72. This shows how pitchers over a larger sample will be closer and closer to the line.

This brings us to the final statistic, K%-xK%. This shows where a pitcher’s been “lucky” or “unlucky,” with high numbers pointing to pitchers that are performing over their heads, and negative numbers pointing to pitchers that should be striking out more batters the rest of the way. All of the data I compiled is in a sort-able table below:

Click the category atop each column to sort by that and click the arrows at the bottom right corner to go to the next page of data. 

As you can see, there is a wide spread of over and under achievers among elite and sub-par pitchers. The highest K%-xK%’s belonged to Jon Lester (7.65%), Yu Darvish (7.38%), and Johnny Cueto (6.24%). All three should remain effective pitchers, but expect their elite K rates to drop a little bit, and Lester in particular could be a good sell-high candidate.

On the other side of the spectrum, some of the pitchers who should see a rise in strikeouts include Francisco Liriano (-5.75%), Hiroki Kuroda (-4.15%), and Justin Verlander (-3.96%), which is great news for their fantasy owners and makes them a good target in a trade. Tyson Ross is another name that pops up on the list, due to his 12.60% Swinging Strike rate, good for 3rd in the league. He is undervalued right now in nearly every league, and is becoming one of my favorite young arms due to his ability to generate lots of strikeouts and ground balls.

This list should be a good way to study strikeout rates across the league and project the pitchers that will improve or decline the rest of the way, but is far from a definitive projection. Some pitchers will continue to show an ability to maintain their extreme rate (be it higher or lower than the average).

Matt Cott is a co-founder of RotoAnalysis. Follow him on Twitter @RotoAnalysis and @KidCotti21!