A few visitors to this site have asked why we no longer include strikeouts per nine innings (K/9) and walks p nine innings (BB/9) in pitchers' statistics tables. The simple answer is that there is no need to; K% and BB%, or the number of strikeouts or walks per batters faced, are better descriptors of pitchers' skill in striking out batters and avoiding walks.

You might be wondering why it matters at all. It's true that both sets of numbers will give you similar information about most pitchers. For some pitchers, however, the differences can be meaningful. Let's take a look at how Kevin Slowey, who is scheduled to make his major league debut tomorrow, compares to other Triple-A pitchers in strikeouts per nine innings:

    Name            K/9
1.  Yovani Gallardo 12.5
2.  Virgil Vasquez  10.6
3.  Jeff Niemann     9.6
4.  J.A. Happ        9.4 
5.  J.P. Howell      9.0
6.  Andy Sonnanstine 9.0
7.  D.J. Houlton     8.5
8.  Matt Garza       8.3
9.  Colby Lewis      8.3
10. Ron Chiavacci    8.3 
11. Jason Hammel     8.3
12. Kevin Slowey     8.0

Slowey is ranked #12 in strikeouts per nine innings. That puts him among the top 25% of qualifying pitchers, but he's still within one standard deviation from the average.
Here are the Triple-A pitchers ranked by strikeouts per batters faced:

    Name             K%
1.  Yovani Gallardo  34.9%
2.  Virgil Vasquez   27.9%
3.  Andy Sonnanstine 25.4%
4.  Jeff Niemann     24.5%   
5.  J.A. Happ        24.4%
6.  Kevin Slowey     23.4%
7.  J.P. Howell      23.4%
8.  Colby Lewis      23.0%
9.  Jason Hammel     22.8%
10. D.J. Houlton     22.8%
11. Matt Garza       21.9%
12. Ron Chiavacci    21.5%

This ranking paints a different picture. Both Slowey and Andy Sonnanstine have impeccable control, and they both move up a few spots when we rank pitchers by K% rather than K/9. Pitchers who do surrender many hits or walk a lot of batters have inflated K/9 rates because they face more batters per inning and therefore have more opportunities to record strikeouts. Six of the eleven pitchers ranked ahead of Slowey according to K/9 are not really striking out batters more frequently than Slowey; they are simply facing more batters per nine innings of work.

K% and BB% do a better job of isolating the skills they measure, so it should come as no surprise that K% and BB% are more strongly correlated from season-to-season than K/9 and BB/9.

If you are more comfortable understanding K/9 and BB/9 than K% and BB%, you can quickly get a feel for the range of these numbers by browsing the league leaders tables. As a general rule of thumb an average professional pitcher will strike out about 15% of opposing batters and walk about 8% of opposing batters.

K/9 and BB/9 will probably persist if only because fans are comfortable using these numbers and 'innings pitched' is a more widely available denominator than 'total batters faced'. But the numbers are available here and many other places as well. There is just no reason to use K/9 or BB/9 when analyzing a pitcher’s abilities.