BABIP Driven Starting Pitcher BB/9 Changers

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.




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Mike Podhorzer produces player projections using his own forecasting system and is the author of the eBook Projecting X: How to Forecast Baseball Player Performance, which teaches you how to project players yourself. His projections helped him win the inaugural 2013 Tout Wars mixed draft league. He also sells beautiful photos through his online gallery, Pod's Pics. Follow Mike on Twitter @MikePodhorzer and contact him via email.

One Response to “BABIP Driven Starting Pitcher BB/9 Changers”

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  1. supgreg says:

    Basically the premise is, the more batters a pitcher faces, the more chances to strikeout/walk hitters.

    Let me play devil’s advocate for a second. If a pitcher gives up less hits, doesn’t that increase the chances for additional innings pitched, with a cap of 9 in most instances?

    In either instance, gaining additional batters either thru giving up hits or pitching more innings, still comes out to facing the same amount of batters in a game. (unscientifically calculated)

    Since I’ve written this out, it makes more sense to me why K/9 or BB/9 can be misleading. Facing 30 batters in 5 IP with a K% of 20% is 6K in 5IP and a K/9 of 10.8. Facing 30 batters in 9 IP with a K% of 20% is 6K in 9IP and a K/9 of 6.

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