Friday, May 29, 2009

Friday Stat of the Day: BABIP

Welcome to the first installment of Friday Stat of the Day, designed to provide a brief introduction to some of the advanced metrics (or concepts really, even if the math isn't that hard) used often by statheads, Bill James-o-philes and blogger geeks (but increasingly more by mainstream media, which is encouraging) to evaluate player performance beyond the standard statistical categories.

We'll kick it off with something that actually isn't too hard to grasp from a numbers perspective but actually can be very helpful in evaluating the current success or failure of a pitcher. BABIP stands for batting average on balls in play, and it essentially measures the percentage of all balls in play that go for hits against a certain pitcher. Home runs aren't counted as a "ball in play," nor are strikeouts obviously, so it largely boils down to all batted balls that a defender would theoretically have a play on. Here's the formula:

where H is hits, HR is home runs, AB is at bats, K is strikeouts, and SF is sacrifice flies.

Seems pretty simple, right? So why is this important?

Basically, looking at a pitcher's BABIP can be used as an indicator that a pitcher is having a fluky season - either a) that they are pitching way over their heads and getting lucky that many of the balls in play simply aren't dropping for hits or that they are benefiting from stellar defense; or b) that they are not pitching nearly as poorly as their win-loss record or ERA (generally regarded by sabremetricians as two of the worst ways to evaluate pitching performance) might suggest, meaning that balls in play are finding more gaps than they normally would or they are being victimized by defensive ineptitude. Extremely high or low BABIPs are not sustainable, and as the short Wikipedia blurb on the subject notes, "those whose BABIPs are extremely high can often be expected to improve in the following season, and those pitchers whose BABIPs are extremely low can often be expected to regress in the following season." Bottom line is that it depends mostly on defense and luck, rather than pitching skill.

So for some real-life examples, let's take a look at the BABIP of a few notable pitchers this admittedly still young season. To keep things in perspective, Baseball Prospectus says that an average BABIP is around .290. I'll highlight a few pitchers at both the top and bottom end of the spectrum.

Getting screwed:
Now here's an interesting example. As of May 21, with a minimum of 30 IP, the pitcher with the highest BABIP was none other than last year's Cy Young winner and (sic) favorite Tim Lincecum, sitting at a hefty .389. Lincecum's line on the season however, is far from a disaster - 3.03 ERA, 4-1 record, 84 Ks in 65 IP. What does this tell us? Number one, the best way to avoid having a high BABIP really hurt you is to miss a ton of bats, at which Lincecum is one of the best. But number two, we should expect his numbers to improve (sick, I know) as his BABIP regresses to the mean.

Following Timmay! at .388 and .382 are Jon Lester and Ricky Nolasco respectively, and although obviously not in the same category as Lincecum, Tim Dierkes notes that they each probably deserve ERAs around 4 rather than the 6.51 and 7.78 they currently sport.

Riding the wave:
One interesting highlight at the other end: Edinson Volquez. Currently sitting at .211. Volquez is putting together a decent season so far, but a lot of people thought that he would regress after a monster year last year. Obviously there are many more factors that go into something like that, not the least of which is probably fatigue from throwing 196 innings at his age, but this is an interesting stat. He's still missing bats at 45K in 48 IP, but his fortuitous BABIP might suggest that tougher times lie ahead. Not to say that last season was all smoke and mirrors, though as his BABIP last year was a very average .303. Make of this what you will.

So in summary, BABIP isn't a perfect, all-powerful or super-predictive stat, but it can help point out some of those guys who are overperforming and underperforming. Check it out for yourself if you wanna know more.

Note: For an interesting analysis of BABIP from a hitter's perspective, here's a good article.

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