Toward the end of last season at the beginning of September, I wrote an article (at my former site) advocating the use of strikeout percentage (K%) over the more commonly used strikeout per nine innings (K/9).
Derek and I continued to have a good discussion on the topic following the article, which kept me thinking about the issue. Is K% an improvement to K/9? Is there another stat, not yet created, that would better show a pitcher’s ability to get strikeouts?
I did not reach a definite answer to these questions, but after playing devil’s advocate in my mind a few times (as I will in this article by the way) I feel I have at least made progress on the answer, which lies in understanding K% and K/9, and their similarities, differences, and flaws.
The biggest difference between K% and K/9 is their baseline. K% is strikeout per batter faced while K/9 is strikeout per inning, which is essentially per out.
A baseline of per out is good because every inning, a pitcher must get three outs. How many hits or walks he allows in that time serves only to inflate the number of batters he faces. He must, however, face three batters that get out. Must. K/9 isolates this, ignoring hits and walks, and shows us how many batters he gets out via strikeouts, holding everything else constant (more or less).
A baseline of per batter faced can also be argued as good because it shows, quite clearly, how often a pitcher can strike a batter out and how often he cannot. It does not matter what the non-strikeout outcome was—be it walk, hit, or ball in play out—if the pitcher could not strike the batter out, they are not as good as someone who could.
Proponents of K/9 could argue that including walks in the K/9 equation is detrimental because control is a different skill that should not be taken into account when trying to determine a pitcher’s strikeout ability. Proponents of K% could counter that walks should be included because they represent a batter that the pitcher could not strike out.
Both stats do have a major flaw, most notably their dependency on BABIP. Consider the following two innings of work:
Ground ball (hit)
Here Pitcher A would have a K/9 of 9.00, as would Pitcher B. Pitcher A’s K%, however, is 33.33 percent while Pitcher B’s is 20 percent.
This certainly leads one to believe that K% wrongly takes into account ball in play outcomes and K/9 is better because it does not. This argument can be flipped onto itself to prove K/9’s dependency on BABIP too, though. Notice how in Pitcher B’s inning of work, one ground ball went for an out and another went for a hit.
Oddly, even though both are ground balls, the outcome of the ground ball—hit or out—determines whether that batter affect’s the pitchers K/9 rate. When the ground ball goes for a hit, the K/9 remains unchanged. But when the ground ball is converted into an out, the K/9 rate will go down because an out was made that was not a strikeout. That does not seem right.
Taking a step back, it seems we have done a good job of pointing out the strengths of weaknesses of both stats. With the flaws both have, I think it is possible to create a new, better stat. To do this, I will take what consider the best of both K/9 and K%.
The per out baseline of K/9 is too illogical, only counting balls in play when they go for outs, and therefore I like the batters faced baseline of K% better. I do like the way K/9 ignores walks, which should be kept separate from the ability to strike batters out.
From these two preferences arise the new stat whose equation is K/(K + BIP) and I will call it True K for now, or TK.
Do I think TrueK is perfect? No. But I do believe it is better at showing who the best strikeout pitchers are. Agree? Disagree? Let me know your thoughts in the comments below