Yesterday, I unveiled another use of a pitcher’s BABIP, besides quickly identifying who has benefited from some good fortune and whose luck should improve. I focused on the K/9 rate and how a low or inflated BABIP could dramatically affect it as it increases or reduces the number of hitters a pitcher will face each inning, giving him additional or fewer opportunities to strike batters out.
Some commenters rightly noted that a pitcher who walks more batters will also have additional opportunities to strike hitters out. This is true, but since walk rate is one of a pitcher’s primary skills, we assume that his BB% will remain fairly consistent. I wanted to isolate the luck component of K/9 and therefore chose to only control for BABIP.
Today I follow the same process, but this time will look at a pitcher’s BB/9. Once again, a low BABIP will reduce the number of batters a pitcher faces in an inning, limiting his chances of issuing a free pass. The opposite would be true with an inflated BABIP.
Like yesterday, I looked at all starting pitchers with at least 50 innings pitched in a season from 2003-2012 (n = 1,770) to formulate a regression equation to turn a pitcher’s walk percentage into an expected BB/9 (xBB/9). That equation is:
xBB/9 = -0.2404 + (BB% * 42.0950)
R-squared = 0.984
The future walk percentage increasers group includes those whose current BB/9 is below his xBB/9. We would expect that this group has a BABIP well below the league average, and it does, with an unweighted average of .274. This is essentially the same as the K/9 increaser group that averaged a .275 BABIP. The decliner group averaged an unweighted BABIP of .319, just a hair below the .323 mark posted by the K/9 decliner group.
Since I am looking at the same population of pitchers with the same BABIP metric, the decliners and increasers list should look similar to the K/9 lists.