A Note on K/BB

Over at Beyond the Box Score, Glenn DuPaul (@Glenn_DuPaul) recently wrote a great article about the flaws inherent to the strikeout-to-walk ratio (K/BB) stat. It’s one you’ve seen in many pitching-related articles here, and one I’ve generally just used unthinkingly lately due to its comfort and ease of use.

DuPaul wrote:

My primary issue with the statistic is the fact that walks are in the denominator. Strikeouts are just as (if not more) important as walks, however when walks are the denominator, a pitcher who is very good at not walking batters, but not a great strikeout pitcher will end up with a high K/BB. It’s simple math.

I prefer to stick with plate appearances as the denominator. Strikeouts minus walks, then divided by plate appearances — (K-BB)/PA — makes for a much more valuable statistic, in my opinion.

For those who feel like that statistic is too much work when K/BB is already calculated right there for use, think of (K-BB)/PA in a different sense.

(K-BB)/PA is the same thing as K% minus BB% (both statistics that are readily available). It’s simply the percentage of batters that a pitcher strikes out minus the percentage of batters that same pitcher walks.

DuPaul then offers a few examples of how K/BB can be a bit misleading, even citing former Phillie Joe Blanton.

Unfortunately, DuPaul’s more technical stat isn’t widely available on FanGraphs and Baseball Reference (yet?) so it won’t be as convenient to cite, but going forward, I will make a concerted effort to use it instead of K/BB. Hopefully the change leads to more accurate analysis going forward.

What follows is a comparison for Phillies pitchers who faced at least 200 batters during the 2012 season.

Strikeout rate (K%), high to low

Player K%
Antonio Bastardo 36.2 %
Jonathan Papelbon 32.4 %
Cole Hamels 24.9 %
Cliff Lee 24.4 %
Joe Blanton 20.5 %
Roy Halladay 20.4 %
AVERAGE 18.7 %
Vance Worley 18.1 %
Kyle Kendrick 17.2 %

Walk rate (BB%), low to high

Player BB%
Joe Blanton 3.2 %
Cliff Lee 3.3 %
Roy Halladay 5.6 %
Cole Hamels 6.0 %
Jonathan Papelbon 6.3 %
Kyle Kendrick 7.3 %
AVERAGE 7.4 %
Vance Worley 8.0 %
Antonio Bastardo 11.6 %

Strikeout-to-walk ratio (K/BB), high to low

Player K/BB
Cliff Lee 7.39
Joe Blanton 6.39
Jonathan Papelbon 5.11
Cole Hamels 4.15
Roy Halladay 3.67
Antonio Bastardo 3.12
AVERAGE 2.51
Kyle Kendrick 2.37
Vance Worley 2.28

(K-BB)/PA, high to low

Player (K-BB)/PA
Jonathan Papelbon 26.1 %
Antonio Bastardo 24.6 %
Cliff Lee 21.1 %
Cole Hamels 18.9 %
Joe Blanton 17.3 %
Roy Halladay 14.9 %
AVERAGE 11.3 %
Vance Worley 10.2 %
Kyle Kendrick 9.9 %

There isn’t a sea change between K/BB and (K-BB)/PA. Perhaps the biggest beneficiary is Antonio Bastardo, who had the third-lowest K/BB of the eight pitchers listed, but moved up to second-best in (K-BB)/PA. Joe Blanton, as mentioned, isn’t quite as attractive in this new light. Compared to the 11.3% league average, though, most qualified Phillies pitchers performed well.