No, not the kind of run differential madness that drives most Jays fans to drink. That kind of differential madness shows the Jays as the sixth best team in baseball (by Third-Order wins), still only good enough for fourth in their own division.
Nope, I'm taking about strikeout-walk differential. We're all familiar with strikeout to walk ratio as a decent measure of a pitcher's worth. The good men at The Book blog (aka Tom Tango) believe K-BB differential has a stronger relationship to both same year success as well as future performance.
One of the biggest troubles with striker per walk is it overvalues eliminating walks relative to adding strikeouts. From the comment section of that very post, we see that if a pitcher strikes out 6 per 9 compared to 3 walks, his ratio is 2. Drop a walk per nine and poof! you're a 3 K/BB and feted as a king. Add one strikeout but keep your walks the same, you're marginally better with a 2.33 K/BB. That just ain't right, especially since guys who miss bats are rack up Ks are harder to come by than guys who limit walks.
Another problem with K/BB is it doesn't stand up to the asymmetry test. If you invert the numbers, the results aren't the same. 3/1 gives a ratio of 3.00, 1/3 gives 0.333. These two numbers aren't the same "distance" from one, which makes them pretty flawed. Another reason home run per fly ball is a much better rate than AB/HR (a ratio.)
Anyway, lets look at the numbers. Which Jays stand to benefit the most for looking at their numbers this way (hint: Brandon Morrow.)
| Name | K/9 | BB/9 | K/BB | K-BB |
|---|---|---|---|---|
| Brandon Morrow | 10.48 | 4.11 | 2.55 | 6.37 |
| Casey Janssen | 8.27 | 2.19 | 3.77 | 6.08 |
| Shaun Marcum | 7.5 | 2.06 | 3.64 | 5.44 |
| Scott Downs | 7.08 | 1.86 | 3.8 | 5.22 |
| Kevin Gregg | 9.66 | 4.53 | 2.13 | 5.13 |
| Marc Rzepczynski | 7.29 | 2.69 | 2.71 | 4.60 |
| Jason Frasor | 8.8 | 4.2 | 2.1 | 4.60 |
| Ricky Romero | 7.65 | 3.26 | 2.34 | 4.39 |
| David Purcey | 7.71 | 3.51 | 2.2 | 4.20 |
| Shawn Camp | 5.67 | 1.83 | 3.09 | 3.84 |
| Brett Cecil | 6.41 | 2.81 | 2.28 | 3.60 |
| Brian Tallet | 6.07 | 4.55 | 1.33 | 1.52 |
| Jesse Litsch | 3.09 | 2.89 | 1.07 | 0.20 |
| Dana Eveland | 4.23 | 5.44 | 0.78 | -1.21 |
Pretty logical stuff. Guys like Gregg and Morrow get away with much, much more than a Jesse Litsch, and this differential method really shows it. By K/BB, Litsch and Tallet don't lag too far behind Morrow and Gregg. A better valuation of strikeouts makes the differences that much more striking.
Thinking about it another way, consider we flipped the rates used. Instead of strikeouts per 9 innings, we used K per batter faced. A pitcher may walk 12% of the hitters he faces but strike out 25%. His K/BB is 2 to 1, just like a pitcher who walks only 6% and strikes out 12%. Using the differential method, we see a much better representation of the qualities of a good pitcher.
If you click through and read The Book post, you'll see that not only are these "the qualities of a good pitcher" as I just wrote, they're also consistent with better pitching performance. Which is good, right?
