Infield Fly Balls and xFIP

Today I saw a couple gripes around the Internets about xFIP and how infield fly balls are not taken into account. On FanGraphs, overall fly-ball percentage is used to calculated a pitcher’s “normalized” home run rate.

This got me thinking about David Gassko’s Batted Ball DIPS article from five years ago where he writes the following about infield fly balls:

Infield flies per ball in play actually have a slight negative correlation with outfield flies per ball in play. Inducing infield flies is a skill, and while it correlates somewhat weakly year-to-year (Lichtman found an “r” of .140), a small subset of pitchers exhibits clear control over the percentage of their fly balls that are infield pop ups. I would encourage studies looking into who those pitchers are—one thing I have noticed is that extreme ground ball pitchers allow fewer than expected infield fly balls.

What I believe is actually going on here is that fly-ball pitchers in general have higher infield fly-ball rates as measured by Baseball Info Solutions. The repeatability of infield fly balls is basically just a side effect of a pitcher’s total fly-ball rate. Looking at all pitchers from 2006-2009, here’s what you get when you bucket FB% in increments of 5%:

FB% Bucket     IFFB%    HR/FB%   HR/OFFB%
< 25%          7.1%     11.1%     11.9%
25% - 29%      7.8%     10.9%     11.7%
30% - 34%      8.9%     10.2%     11.2%
35% - 39%      9.7%     10.2%     11.3%
40% - 44%     10.5%     10.0%     11.2%
45% - 49%     11.6%      9.8%     11.0%
>= 50%        12.2%     10.0%     11.4%

So, while it’s pretty clear that overall FB% is impacting IFFB%, I’m not sure things are quite so obvious with home runs. It seems to me that home-runs-per-total-fly-ball plateaus at about 10% starting in the 30%-plus range. And for home-runs-per-outfield-fly-ball, things look pretty similar, except everything is about 1% higher because of the removed IFFBs.

So getting back to xFIP, does it really matter whether or not you exclude popups? The answer is, not really. You’re going to get almost the same results because HR/OFFB on average exhibits more or less the same issue as HR/FB. In fact, the correlation between using OFFB vs total FBs in xFIP is .996. The two, in practice, are virtually identical.

However, when you bucket the data like this, it seems that there is one thing made clear: When an extreme groundball pitcher induces a fly ball, there’s slightly greater chance it will end up a home run. I think it would be particularly interesting to look at the run values of different batted balls types for different buckets of fly-ball pitchers, but I’ll have to leave that for another time.

We hoped you liked reading Infield Fly Balls and xFIP by David Appelman!

Please support FanGraphs by becoming a member. We publish thousands of articles a year, host multiple podcasts, and have an ever growing database of baseball stats.

FanGraphs does not have a paywall. With your membership, we can continue to offer the content you've come to rely on and add to our unique baseball coverage.

Support FanGraphs




David Appelman is the creator of FanGraphs.

newest oldest most voted
jeffrey Gross
Guest
jeffrey Gross

HR/FB is a useless baseline for xFIP because everyone knows that an IFFB can’t be a HR, but you are including it in the data from which to derive predictive material. It’s making xFIP less correlative for no justifiable purpose other than “we’re too lazy to change it.” Until you run the numbers and demonstrate that the adjusted R-squared of pitcher’s xFIP based on HR/OFFB is a statistically insignificant variance from xFIP based on HR/FB, I will continue to use HR/OFFB, please do not assert that “you’re going to get almost the same results”

Fangraphs is the place where baseless claims go to die, not be perpetuated. Vive le HR/OFFB.

jeffrey Gross
Guest
jeffrey Gross

Side Note:
maybe this is a glitch, but my comments are not registering on the site when my blog is listed under “website.” anyone else finding this problem, or have I vexated the wrong persons?

jeffrey Gross
Guest
jeffrey Gross

Oh and apologies if it seems like that last sentence of my org. statement was overly critical and “kinda douchey.” My internal brain filter does not properly function at 130 am. I obvious love the site and material herein.

philosofool
Member
Member
philosofool

IFFB aren’te a repeatable skill (r = .14, r^2 = .02). The point of xFIP is to remove luck. If you don’t want to remove luck, use RA/IP.

Nick Steiner
Guest

IFFB aren’t a repeatable skill? I highly disagree with you on that, but regardless, why would you use them in HR/FB at all? xFIP is a descriptive stat (meaning it uses raw, unrepressed data) so IFFB should definitely be excluded from the calculation, even if there will hardly be a difference with and without them,

neuter_your_dogma
Guest
neuter_your_dogma

“an IFFB can’t be a HR.” Same is true for any FB that is not a HR. Why would FBs hit 10 feet past the infield count and IFFBs would not?