Sabermetric Mining: SwStr%, K%, and K/9

Posted on 17 August 2012 by Blake Murphy

Strikeouts are not always the most obvious statistic to predict. While velocity, movement, and deception can all play a part, there are still instances where soft-tossers manage to get strikeouts or flamethrowers struggle to do the same. While there is no catch-all metric to help predict future strikeout performance, fantasy players can use Swinging Strike Percentage (SwStr%) as a backwards-looking tool to see which pitchers have over- or under-performed relative to their strikeout expectations. Thus, SwStr% can help players identify potential breakouts or downturns in the ever-important ‘K’ category.

The Stat
SwStr% – Simply put, this stat is the percentage of pitches a pitcher throws that a batter swings at and misses. Mathematically, it is just “swings and misses” divided by “total pitches thrown,” and it acts as a good proxy for a pitcher’s dominance level. While pitchers can generate looking strikes as well, swing and miss pitches are a better indicator of dominant performance.

K% – this is the percentage of at bats that end in a strikeout, or, K%=K/AB. While it is not the statistic used in fantasy leagues (usually just strikeouts, or perhaps K/9), it is a good indicator of strikeout ability in batter-comparable terms.

K/9 – Strikeouts per nine innings, or K/9=K/(IP*9). This statistic can be more useful than strikeouts alone, especially in leagues that use innings limits, as it allows owners to identify pitchers who strikeout many or few batters, isolating for the amount of innings they pitch. For example, if you are nearing your innings limit, a pitcher with a 9.0 K/9 but with fewer anticipated innings the rest of the way may be more valuable to you than a pitcher with a 7.0 K/9 expected to have a heavy workload down the stretch.

As a rough estimate, a K% of 25% will lead to approximately a 10.0 K/9, while a 10% K% will lead to approximately a 4.5 K/9, with the points between trending together as you would expect.

How To Use
At the end of last season, Bradley Woodrum at Fangraphs looked at how Swinging Strike rate relates to K%, or the percentage of at bats that end in strikeouts. He found that, while the error terms vary quite a bit, strikout percentage and swinging strike percentage correlate pretty strongly (.6928 R^2 value, or, stated otherwise, swinging strike rate can explain about 69% of the variance in strikeout percentage). Woodrum’s regression allowed me to create the following chart, which can act as a rough approximation of the K% we should expect at given SwStr% levels.

Using this chart and standard leaderboards, we can try to identify pitchers who have a high SwStr% but a low K% or a low SwStr% but a high K%. Those with a high SwStr% and a low K% should be expected to strikeout a higher percentage of batters than they have so far, and vice versa.

Edwin Jackson – Jackson has an impressive 11.7% SwStr%, tied for the 6th best mark in the league, but boasts just a 19.3% K%. Our chart above indicates that an 11.7% SwStr% would be more in line with a 24% K%, which would greatly help Jackson improve on his 7.23 K/9 mark. In fact, based on SwStr% alone one would expect Edwin to strikeout about a batter an inning. While this might be high as an expectation moving forward, based on other factors in his profile that may impact strikeout proficiency, we can safely anticipate some measure of uptick in strikeouts for the remainder of the year.

Vance Worley – Worley has somehow managed to put up strikeout rates in line with Jackson despite causing far fewer batters to whiff. His 5.5% SwStr% would indicate an expected K% of about 13%, but Worley checks in at 18.8%. Either Worley has found a way to make batters only miss on third strikes, basically “saving up” his good stuff, or he has been a little lucky and probably won’t be striking out more than six batters per nine innings moving forward.

Gio Gonzalez – While nobody will argue Gonzalez’s dominance this season, his strikeout proficiency has been a but overstated based on his SwStr%. His SwStr% of 9.6% is more in line with a K% of 21% and a K/9 of 8.25 rather than his marks of 25.8% and 9.63. He is still an ace, but those relying on him for seven or eight strikeouts a start might be slightly disappointed down the stretch.

Candidates
I should note here that the ‘candidates’ section this week is a bit thin because I had to pick arbitrary endpoints. Really, you would want to download the leaderboards and create an “Expected K%” column and compare that to actual K%. These are just a few of the more extreme examples.

Potential Sell High – These pitchers have a SwStr% of less than 7% but a K% of greater than 15%, indicating a potential decline in strikeout rate forthcoming.

Potential Buy Low – These pitchers have a SwStr% of greater than 10% but a K% of less than 21%, indicating a potential increase in strikeout rate forthcoming.

Strikeouts can be a difficult statistic to predict as the season rolls along. Sometimes, the stats do not match what we see with our eyes, or curiosities defy logic like Aaron Cook not being able to generate a swinging strike if I was batting. While more goes into the art of the strikeout than just causing batters to whiff and any pitcher worth his salt can attest that there are a dozen factors that go into a strong SwStr%, SwStr% remains a strong metric for aiding fantasy owners in identifying potential value in the K category.

Come get to know me on Twitter @BlakeMurphyODC.

All stats courtesy of FanGraphs, for games through August 15.

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