Better Know a Statistic: (K-BB)/PA

In Commentary And Analysis, theory by myles10 Comments

I have a confession to make. 

When I first fell headlong into baseball analysis, I kind of took K/BB as my pet statistic. I loved how it was essentially "luckless": the only things the pitcher couldn't control was the discipline of the batter and the strike zone of the umpire, the two things he can only understand (but never master). Not only was it good at explaining the two fundamental pitching statistics, it did it in a way that aped the GPA system in the American school system. If you had a 4.00 GPA, you were an excellent student: a 4.0 K/BB meant you were an excellent pitcher. 

After a while, though, I began to have my doubts. Is a pitcher that strikes out 9 per 9 innings and walks 3 in the same span really only half as good as the guy that strikes out 6 and walks 1? If we assume a normal BABIP (lets say .300 exactly), these two pitchers profile thusly (over a 27 out span):

Pitcher A: 9 strikeouts, 18 regular outs, 7.7 hits, 3 walks, 10.7 baserunners

Pitcher B: 6 strikeouts, 21 regular outs. 9 hits, 1 walk, 10 baserunners

Pitcher B has a 6 K/BB, but allows nearly as many baserunners as Pitcher A does. Not only do they allow baserunners at close enough a rate as makes no difference, Pitcher B actually allows more active baserunners. I'd expect Pitcher B to allow more runs per 27 outs than Pitcher A does. 

This sort of trickery is what allows Joe Blanton to accrue such gaudy K/BB totals while being a mediocre pitcher at best.

2005 Athletics 5.19 3.00 1.73 1.03 13.9 % 8.0 % .233 1.22 .248 75.3 % 83 105 4.43
2006 Athletics 4.96 2.69 1.84 0.79 12.5 % 6.8 % .304 1.54 .335 68.9 % 108 94 4.16
2007 Athletics 5.48 1.57 3.50 0.63 14.7 % 4.2 % .265 1.22 .299 68.0 % 90 81 3.50
2008 2 Teams 5.05 3.01 1.68 1.00 13.0 % 7.7 % .269 1.40 .290 68.4 % 110 106 4.52
2009 Phillies 7.51 2.72 2.76 1.38 19.5 % 7.1 % .257 1.32 .291 78.9 % 96 104 4.45
2010 Phillies (A) 9.00 0.00 2.00 0.00 33.3 % 0.0 % .000 0.00 .000 100.0 %     1.20
2010 Phillies (AA) 5.63 2.25 2.50 2.25 14.3 % 5.7 % .273 1.38 .269 73.2 %     5.95
2010 Phillies 6.87 2.20 3.12 1.38 17.5 % 5.6 % .287 1.42 .321 69.1 % 120 108 4.34
2011 Phillies (A) 0.00 0.00 0.00 0.00 0.0 % 0.0 % .000 0.00 .000 100.0 %     3.20
2011 Phillies 7.62 1.96 3.89 1.09 19.4 % 5.0 % .306 1.48 .362 70.9 % 131 94 3.63
2012 2 Teams 7.82 1.60 4.88 1.37 20.6 % 4.2 % .269 1.26 .310 67.9 % 121 101 3.91

Clearly, K/BB isn't actually providing a meaningful statistic.

What matters in pitching is acrruing as many outs as you can in the fewest number of batters you can. All of the other variables are going to average themselves out, somehow. 

If you take as a given DIPS theory, that a pitcher has no control over balls in play, it stands to reason that the same pitcher has little or no control over the distribution of the hits that take place 30% of the time. Therefore, the best way to reduce runs is to strikeout as many batters as possible while denying as many free passes as you can – the primary reason that strand rate is also essentially volatile. K/BB isn't a measure of this skill at all.

What is, however, is K-BB. The difference between strikeouts and walks is the difference between batsmen and batted balls. This is a much better statistic, because it describes an actual skill in reducing runs – coincidentally, the only skills he can actually control.

The only thing that's left is converting this count statistic into a rate statistic. The best way to do that is to take this difference in batsmen/batted balls and set it to actual batters faced. Thus, (K-BB)/PA is the statistic that best addresses this problem.

Let's go back to Pitcher A and Pitcher B.

Pitcher A: 6/28.7 = 20.9%

Pitcher B: 5/31 = 16.1%

This gives us a clearer view of the value of these two pitchers. Pitcher B has the far superior K/BB, but he opens himself up to more batted balls, and as such is worse (albeit only slightly worse).

The one concern with this statistic is that it doesn't take HR% into account. First, that's also a rebuke to the old stat (K/BB). More importantly, though, HR% is actually a fairly erroneous statistic in itself. It's open to all sort of disputes: is a HR in Coors Field the same as a HR in Petco Park? HR% is a "pitcher's skill" to some effect (home runs imply hard contact), but it's much, much less useful than K/BB.

If you have any questions about either stat, feel free to comment.

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  1. dmick89

    Yeah, strikeouts and walks is really the best way to evaluate a pitcher. K-bb% is probably my favorite and wish Fangraphs or bref would add it.

    That reminds me, I’ve been meaning to start using kwERA around here, but keep forgetting. Need to set up a spreadsheet so I can do that more easily this season.

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  2. Rizzo the Rat

    K/BB isn’t a bad stat, even if per PA approaches are better. Joe Blanton isn’t much of an exception, since his career K/BB isn’t spectacular (his long-ball tendencies and high BABIP explain why his ERA is higher than you would expect from K and BB alone). What’s really interesting (to me) is the pitchers who can succeed in spite of their K/BB ratio (or K-bb%). Tom Glavine and Jim Palmer didn’t have very good control and didn’t strike a lot of people out, but somehow managed to have great careers.

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  3. SVB

    After round 1 of the NCAA tournament, there is one team left at every seeding level except 16. Wonder when that happened last, if ever.

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  4. Rizzo the Rat

    dmick89 wrote:

    Yeah, strikeouts and walks is really the best way to evaluate a pitcher.

    Only in the short term. Otherwise, we’d have to conclude that Tom Glavine wasn’t very good, despite pitching a ton of innings without allowing many runners to score. (Actually the best way to evaluate a pitcher over any period of time is to regress everything, but K and BB regress less than other things and get you more than halfway there.)

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  5. dmick89

    @ Rizzo the Rat:
    There are exceptions to almost everything. What I meant is that looking at those two stats will, most of the time, tell you who is good and who isn’t. I also agree with what you said about Glavine and Palmer.

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