K% vs. K/9: Finding an Effective Metric for Strikeout Rate

Chicks dig the long ball, but pitchers prefer the strikeout. When measuring pitcher performance, strikeouts are often a key factor in the equation. Unfortunately, however, the equation is tragically flawed—the K/9 equation, that is.

Long used as the standard metric for strikeout rate, K/9 often appears alongside other “fundamental” stats used in broadcasts, such as ERA and win-loss record. While many have embraced superior alternatives to ERA and pitcher wins, K/9 remains stubbornly lodged in the contemporary baseball lexicon.

The equation for K/9 is simple; the name of the metric itself is the equation! That is, K/9 = total strikeouts/(total innings pitched/9).

At first blush, K/9 seems rather arbitrary—why use a complete game for the baseline rather than a single inning pitched? This is really only a minor quibble, as the sabermetric community is far more concerned with K/9’s glaring deficiency: a dependence on batted ball outcomes.

Because K/9 weights strikeouts against outs recorded, batted ball outcomes play a considerable role in determining a pitcher’s K/9. Pitchers who allow base runners, whether through poor pitching or bad luck, see more opportunities to record strikeouts. As a result, their K/9 rates can become artificially inflated. Consider the following thought experiment:

In the top of the inning, pitcher A strikes out the first batter he faces. He hits the next batter with an errant pitch. The next two batters hit back-to-back home runs. A blooper falls in front of the right fielder. Pitcher A strikes out the next hitter. He then issues a walk and gives up a double. Pitcher A finally gathers himself and collects the last out via a fly ball to the left fielder.

In this scenario, pitcher A faces nine total batters and manages to record two strikeouts. His K/9 sits at a cool 18.00.

Contrast pitcher A with pitcher B, who strikes out the last batter he faces in a clean 1-2-3 inning. Pitcher B, who strikes out one third of the batters he faces, owns a 9.00 K/9 that pales in comparison to pitcher A’s 18.00. Pitcher B earns a K% of 33%, however, while pitcher A carries a K% of 22%.

K%, which is measured as total strikeouts divided by total batters faced, provides a simple solution to K/9’s deficiencies. K% takes batted ball outcomes out of the equation entirely. No longer is a pitcher’s strikeout rate artificially inflated by bad luck or poor command. Each batter faced–each opportunity to record a strikeout counts toward a pitcher’s K%.

I gathered K/9 and K% data from the past season to discover which players benefited and suffered the most from K/9’s errant scoring method. After finding the mean and standard deviation for 2014’s starting pitcher K/9 and K%, I compared each player’s z-scores to discern which pitchers’ strikeout abilities were most overrated and underrated by K/9. Consider:

2014’s Top 10 Most Overrated Pitchers by K/9:

Rank ▾NameK/9K/9 z-ScoreK%K% z-ScoreDifference
1Travis Wood7.570.0950.187-0.300-0.396
2A.J. Burnett80.4080.2030.096-0.312
3Francisco Liriano9.71.6420.2531.335-0.307
4Jake Odorizzi9.321.3660.2421.062-0.304
5Colby Lewis7.03-0.2970.175-0.598-0.301
6Zack Wheeler9.081.1920.2360.914-0.278
7Drew Hutchison8.971.1120.2340.864-0.248
8C.J. Wilson7.740.2190.198-0.028-0.247
9Wade Miley8.180.5380.2110.294-0.244
10Ian Kennedy9.271.3300.2451.137-0.193


For the most part, these guys fit the profile of pitchers who might benefit from K/9’s blind spots. Travis Wood is a very hittable pitcher with sub-par control. Likewise, Francisco Liriano and AJ Burnett have never been known for their pinpoint control. During his first full season, Zack Wheeler struggled to find the strike zone. This is not to say that some of these pitchers are poor strikeout pitchers; Liriano, Odorizzi, Wheeler and Kennedy all posted a K% roughly one standard deviation above the mean. By K/9’s standards, however, they are overrated to varying degrees.


2014’s Top 10 Most Underrated Pitchers by K/9:

RankNameK/9K/9 z-ScoreK%K% z-ScoreDifference
1Clayton Kershaw10.852.4770.3192.9700.493
2Felix Hernandez9.461.4680.2721.8060.338
3Adam Wainwright7.1-0.2460.199-0.0030.243
4Josh Collmenter5.81-1.1830.161-0.9450.238
5Doug Fister5.38-1.4950.148-1.2670.228
6Hisashi Iwakuma7.740.2190.2170.4430.224
7Johnny Cueto8.941.0900.2521.3100.220
8Tanner Roark6.25-0.8630.173-0.6470.216
9Chris Sale10.762.4120.3042.5980.187
10Dallas Keuchel6.57-0.6310.181-0.4490.182


That’s a fun list. Clayton Kershaw, Felix Hernandez and Adam Wainwright are all elite in terms of limiting base runners. Their strikeout totals are more meaningful because of how few batters they faced in the process of accumulating them. Josh Collmenter, Doug Fister and Hisashi Iwakuma all possess excellent command. The guys toward the bottom of the list are no slouches either—while Chris Sale and Johnny Cueto are dominant, Tanner Roark and Dallas Keuchel are highly effective.

Sometimes it’s difficult to box up old metrics and move them to the metaphorical attic. Although the K/9 statistic carries a certain element of nostalgia, it’s time to move forward and embrace a much more meaningful measure of strikeout efficiency: K%.

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