Cahill and BABIP

There are a lot of good comments in this afternoon’s post about how we should evaluate pitchers for the Cy Young Award. We’ll get into more detail about the FIP/WAR discussion tomorrow, when I have more time than I do right now to really talk about the issue in some depth.

One comment that keeps arising, however, is about the correlation between Trevor Cahill’s BABIP and his sinker, specifically his ground ball rate. Several people assert that Cahill is inducing weak, easy to field contact by pounding his sinker at the bottom of the strike zone, and that’s why his BABIP is just .217. There are a few problems with this assertion, though.

We know that BABIP on groundballs is higher than on flyballs, as a ball is more likely to sneak between two infielders than it is to fall in front of an outfielder. In general, groundball pitchers will post higher than average BABIPs, not the other way around, though the effect is generally pretty small.

The other problem… well, we’ll just demonstrate it this way.

Trevor Cahill: 56% GB%, 14.9% LD%, 29.1% FB%, .217 BABIP
Justin Masterson: 62.3% GB%, 14.9% LD%, 22.8% FB%, .344 BABIP

The argument that this particular skillset is the driver of a low batting average on balls in play falls apart when you consider that Masterson, who gets more groundballs and has an identical line drive rate, is posting one of the highest BABIPs in all of baseball. We cannot just see two variables and assume that one is the cause of the other. Cahill has a high groundball rate, and he has a low BABIP, but there’s just no evidence that the former is driving the latter.

The line drive rate is the real factor here. Among the nine starters who have a LD% under 15 percent, the average BABIP is .271, well below the league average. As you probably know, the lion’s share of hits in baseball come on line drives, and so a pitcher who doesn’t surrender that many hard hit balls will also not allow that many hits (though, this does not appear to be a skill, as the year to year correlation of LD% is very low).

Again, though, Cahill’s BABIP stands out as a crazy outlier, even in this no-line-drives group. If you take him out of the sample, the average BABIP for the remaining eight guys is .278, sixty points higher than Cahill’s, even though he’s at the high end of line drive rate for this subset of pitchers. Even if we also throw out Masterson to even things out, the other seven guys have a BABIP of .268, still way higher than Cahill’s mark. And, again, they have lower line drive rates than Cahill does.

Cahill is likely throwing pitches that are harder to hit than an average pitcher. He deserves some credit for that, even if he can’t keep it up. However, on top of that, he’s almost certainly just getting some good fortune, whether it be through assistance from his defense or just lousy hitting from his opponents. We cannot, and should not, give him credit for the .217 BABIP just because it happened. It isn’t all him.