If you haven’t already, go read David Laurila’s Q&A this morning with Dan Rosenheck, writer for the Economist and New York Times, who gave a presentation on predicting BABIP at the Sloan Conference last week. In that piece, Rosenheck notes that he created a model using just two variables — infield fly rate and rate of contact on strikes — that helped explain 15% of the variance in a pitcher’s future BABIP. The part about infield flies helping reduce BABIP has been noted before, as others have created takeoffs of ERA estimators that incorporate batted ball data — SIERA, tRA, bbFIP, etc… — and Steve Staude wrote a Community Blog post on this topic back in October, also identifying infield fly rate as a significant explanatory tool for BABIP. The potential explanatory effects of inducing popups and the link to Z-Contact% is fascinating, however, and makes Rosenheck’s study a real step forward.
It makes perfect sense that infield flies would help explain some of the variation in a pitcher’s BABIP, of course, since infield flies are almost always outs. In fact, in 2012, there were 4,377 batted balls that were categorized as infield flies in Major League Baseball, and only 13 of those went for base hits. Another 28 did not result in an out due to an error by the fielder, but even with 41 non-outs, that leaves IFFBs with an out rate of 99.1%.
Infield flies are, for all practical purposes, the same as a strikeout. They are basically an automatic out, runners do not advance on infield flies, and perhaps most importantly, we can state with a pretty high level of confidence that the relative abilities of the defenders have nothing to do with the outcome of the play. Sure, maybe you or I wouldn’t turn every IFFB into an out, but for players selected at the Major League level, there is no real differentiation in their ability to catch a pop fly.
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