Wednesday, July 22, 2009

A note from a reader on BABIP

It has been some time now since the BABIP established its place in the sabermetric canon, and understandably so. BABIP means to aim a quantified scrutiny at a rather interesting problem: the extent to which a batter’s skill at directing 90 mile-per-hour pitches away from the best preventative efforts of sagely-positioned, gloved men can be assayed.

Yet, I have long experienced a measure of unease with the BABIP, and while my hermetic disquiet won’t suffice to give pause to a sabermetrician—and it shouldn’t—perhaps this simple example will:

Imagine two hitters, A and B, both of whom assemble 600 at-bats, 180 hits, and 100 strikeouts. A hits 15 home runs, however, while B musters 40. Both players hit .300, of course, but their respective BABIPs look like this:

A -- .340
B -- .304

And therein lays the conundrum. In what manner shape or form are we entitled to conclude, on the basis of the evidence placed before us, that it is A who more adeptly interposes batted balls between the defenders assigned to thwart him? How can we possibly formulate such a judgment when the data affords us no warrant to do so?

All we know is that B outhomers A, and all we have is a metric

(Hits-HR)/(AB-HR-SO+SF)

which falsely skews its conclusion. And that is because by informing both tiers of the fraction, the subtracted home runs pare the numerator artificially for power hitters, culminating in lower BABIPs.

Again - why should we be entitled to thus declare that high-home run achievers commit inferior skills to the challenge of turning balls in play into hits? That inference is simply not available to us.

In fact, BABIPs typically exceed players’ averages ; but for big power hitters, the relationship undergoes a curious inversion. Babe Ruth’s .342 lifetime average turns into a BABIP of .340. For Hank Aaron, the split stands at .305/.295; yet Rod Carew checks in at .328/.361. Can we thus assert, with the appropriate, straight-faced measure of confidence, that Carew’s ball-in-play facility truly overwhelms Aaron’s by 66 points?

The inarguable mathematical point is this: that, all other things being equal, the player with more home runs suffers a relative decrement in BABIP. All else is speculation. If we proceed from the eminently clear-eyed premise that home runs tend to be hit harder than the average ball in play, we can go on to propose that, were these fence-clearers to fall short by a few feet, they would nevertheless fall for hits in greater profusion than typical batted balls - thus resulting in a higher projected BABIP for power hitters.

That too qualifies as conjecture, albeit a sensible one. Still, the fact is that the BABIP puts the caliper to a most intriguing property of the batter’s skill set in a manner that doesn’t quite measure up.

And thanks for sharing my unease.


1 These figures omit sacrifice flies, which contribute a very small effect.