BABIP Splits

Ever since Voros McCracken’s DIPS theory came to light in 1999, people have begun to look at a pitcher’s batting average on balls in play, or BABIP. As Voros noted, variations in BABIP from league average regress heavily to the mean in future years, and it’s value as a predictive measure is quite low. This insight helped paved the way for things like FIP and evaluating pitchers by the outcomes they can control and a movement away from metrics such as ERA.

As more research was done, though, it was found that BABIP isn’t entirely random. Knuckleballers have significantly lower BABIP than a traditional pitcher. Left-handers tend to have some minor BABIP advantage, as do flyball pitchers (though what they save in BA they give back in SLG). However, when looking through the major league splits pages on Baseball Reference, I noticed one other type of pitcher that has a significant BABIP advantage – the home team pitcher.

Here’s a chart to illustrate what I’m talking about.

hrbabip

In every year from 1995 to 2008 (and probably before – I didn’t bother going back any further once I found this obvious of a trend), the batting average of balls in play allowed by the home team’s pitchers was lower than the road team’s pitchers. The two lines generally move together, so when league BABIP is up or down, it’s up or down for both home and road in proportional amounts. But the home line never crosses the road line. It gets close in 2004, when the gap is just two points, but then diverges back to the more normal five to 10 point spread.

Over that 14 year period, home team BABIP allowed is .295, while road team BABIP allowed is .302. We’re talking millions of plate appearances here, so a seven point spread is certainly significant. It’s essentially impossible for this to happen randomly. There is something inherent to being the home team that allows you to reduce the amount of hits you allow on balls in play. This is, for lack of a batter term, a home field advantage.

What could be causing this spread in BABIP between home and road pitchers? Isolating a single factor is going to be next to impossible, and in reality, it probably isn’t a single factor. Outfielders learn how to read the ball off the bat in a specific lighting based on repetitive experience. Infielders learn how the grass makes a ball spin at different speeds. Pitchers figure out where the ball carries and where it doesn’t and pitch away from the areas that can hurt them the most. Hitters pick up the ball coming out of the background quicker. GMs acquire players who fit the quirks of their specific ballpark. It could be any of these, none of these, or all of these.

But we know this – there’s a distinct advantage in being the home team in turning balls in play into outs. If a pitcher gets an inordinate amount of home starts, we shouldn’t be surprised if he beats his career BABIP.




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Dave is a co-founder of USSMariner.com and contributes to the Wall Street Journal.


39 Responses to “BABIP Splits”

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  1. Matthew Carruth says:

    Great catch, Dave! That’s interesting stuff.

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  2. Sean S says:

    I wouldn’t be surprised if this had more to do with the hitters than it did the pitchers/fielders. It seems to me that for many hitters being comfortable with the batter’s eye is huge, and it’s hard to doubt that many players tailor their approaches to the park in which they get the most ABs.

    I would be interested in seeing how the GB/FB/LD splits are affected by being at home or away. If the effect is more a result of the batter’s relationship with the home park, I’d think we’d see an uptick in LDs. If it’s a result of the pitcher’s/fielders’ relationship, I’d expect to see the batted ball rates stay similar while the DER for each category went up.

    That’s a great observation though, Dave.

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  3. I’m not sure how you guys calculate BABIP (do you include ROE?) but could there be a possibility of teams’ scorers being more lenient at home and dishing out more errors for the home team’s, thus lowering the pitcher’s BABIP some?

    Probably not 7 points, but maybe contributes to it.

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    • The Ancient Mariner says:

      Would you really call that “lenient”? I’d call it “pinning more of the blame on the fielders,” myself.

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    • don says:

      I don’t know if that makes sense. Sure, they don’t want to break up a no hitter or something like that. But does the home team really want to give its own fielders more errors?

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      • Sky says:

        Yes. Errors lead to fewer earned runs for pitchers. And there’s motivation for a home team’s scorer to call fewer errors on the away team’s fielders: more hits for the home team batters.

        How many BIP are there per team per season? How many hit/error switches are needed to close this gap?

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      • don says:

        So the scorers favor the pitchers over the fielders? I guess that makes sense, since errors are a less visible stat, but you’re still screwing some of your own guys in favor of others.

        Someone can give a better average if they want but it looks like most teams had something like 4000-4500 balls in play last season (just eyeballing ABs – Ks – HRs) so .007 of that would be an extra ~30 errors over the course of a season.

        However, home teams also hit more home runs, win more games, and score more runs than road teams so even if the BABIP difference is due to the scorer there are some advantages to hitting at home.

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  4. it’s stuff like this that makes this one of my favorite new websites. glad i found it.

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  5. Hizouse says:

    My first thought was similar to Graham’s: home team scorekeeper is more generous to home hitters than to visiting hitters. I would think this would be pretty easy to test: is there a difference between the number of errors for visiting teams versus the number of errors for home teams?

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    • Dave Cameron says:

      The ROE difference is pretty small. We’re talking 2.5 more ROE for visiting pitchers per 3,000 PA (which is about one team’s home or road season’s worth of batters faced).

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      • Hizouse says:

        Thanks. That is in fact the opposite of what I would expect–more errors for home fielders would mean more ROE for home pitchers.

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  6. Hizouse says:

    and by the way, great work, Dave.

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  7. Kincaid says:

    There is some difference in batted ball types for some stadiums at least. For example, Coors has a homefield advantage in line drives over the past 5 years at least, probably due in part to the home hitters being more used to how pitches break differently in altitude. Other factors would play into it as well, like the batters eye, etc. I would still think a pretty good chunk of this is familiarity for the fielders as well, though. It would be interesting to see more work on this.

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  8. alskor says:

    Very interesting… Certainly a factor that is overlooked in pitcher evaluation and awards.

    The thought occurs immediately that most players hit better at home… and we dont have an exact answer of why. It certainly makes sense that players defend better at home, in a familiar park, sleeping in their own beds, etc…

    Defensive efficiency being the converse of BABIP this interests me on that level as well – players are much better defenders in their home parks then away. So… if that holds true it certainly helps explain the lion’s share of why there is a home field advantage.

    My next question would be if you found these results were consistent among all parks… or if a couple teams with a strong home field advantage (ie. the Twins on turf) were skewing things slightly.

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  9. Sean says:

    Geez Dave, you are good.

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  10. John says:

    One word: Metrodome

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  11. fifth of says:

    I wrote about this for THT in 2005, breaking it down by GB/LD/FB based on six years of STATS data: http://www.hardballtimes.com/main/article/the-home-field-advantage/

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  12. Fresh Hops says:

    Here’s a potential explanatory factor that’s weird and cool:

    BABIP in innings 7-9 is lower than BABIP in innings 1-6 by about .010. Since the home team hits in the 9th only about half the time, that should be a partial explanation of the phenomenon. I don’t think it’s enough to be a total explanation, but a partial one. The late inning/early inning split is about .010 but the 9th only makes up 1/9 of the game, so we would expect that the quality of hitting in the ninth would make up a pretty small amount of the difference, between .001 and .002 of the difference. Mostly I was just pointing this little BABIP split out because I thought it would be interesting.

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    • robbbbbb says:

      Not only that, but the home team doesn’t hit in the ninth in a game that they’re leading. Consequently, they’ve (usually) had a better hitting performance than the road team does. That’s a confounding factor, but I don’t know how much of one.

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  13. Xeifrank says:

    This phenomenom is something I’ve studied a little bit. If you really want to see the difference between home and away hitting the biggest difference comes in terms of how often a home team hits a triple, over how often an away team hits a triple. The home team also has a significant advantage in the fact that their hitters walk more often and strike out less often. It’s not just a batted ball phenomenon. Home run rates only slightly favor the home team, with a year or two when the away team actual had a higher HR rate.
    vr, Xei

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  14. Guy says:

    The biggest home advantage is K/BB ratio. One ancillary effect may be that road hitters are a bit more likely to put balls in play on pitchers’ counts rather than hitters’ counts. And BABIP is higher on hitters’ counts. So that could be one piece of the puzzle. I’d be surprised if a home fielding edge weren’t part of the story as well.

    I’m hoping someone will look at the pitch/fx data some day to determine whether the K/BB effect is a) better home pitcher control, b) worse strike zone judgment by away hitters, c) crowd-induced umpire bias, or d) all of above.

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  15. salb918 says:

    DE-FENSE!

    My guess is that this has to do with defenses. Put another way, it’s not that home pitchers have a lower BABIP, it’s that home defenses have a higher DefEff.

    One way to test that would be to look at the BABIP for outfield flies versus infield grounders. If my hypothesis is correct, there should be little difference for infield grounder BABIP between the home and road pitcher, but there would be a difference for outfield fly BABIP between the home and road pitcher.

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  16. Guy says:

    Sal: If you check out Tom M’s piece linked above, you’ll see it’s the opposite: big home/road difference on GBs, very little on air balls. But I don’t think that’s inconsistent with a fielding explanation: home infielders seem to have an edge. It’s not clear that home OFs, except in preventing triples.

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    • salb918 says:

      Guy – thanks for the link. Don’t know how I missed it.

      So, there may be a home field advantage for infield grounders. As you noted, it need not exclude the fielding explanation. But one could argue that the pitcher is “giving up” easier to field ground balls, if there is such a thing.

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  17. Fresh Hops says:

    K/BB ratio doesn’t measure anything relevant to run scoring. A 9K/9 and 3BB/9 pitcher has the same ratio as a 3k/9, 1bb/9 pitcher. The former is a top of the rotation starter, the later comes to camp every spring hoping to find a way to keep his job. Ks and BBs should always be treated as separate performance indicators and measured separately, even though in principle they cannot be independent of one another.

    I hope that didn’t come off too snarky. I have a gripe with K/BB ratio as a performance indicator.

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  18. Brian Cartwright says:

    I saw this effect when doing WOWY on defense – fielders looked like they consistently had a slight advantage at home. This was looking at outs, so roe was included. I gad assumed this was a function of defense, perhaps the fielders being more familiar with and thus do better on their home field. This has been suggested as a reason for home field advanatge in some recent studies, one I believe by Joe Sheehan at BP.

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  19. Guy says:

    “K/BB ratio doesn’t measure anything relevant to run scoring.”

    An overstatement, but I take your point. The ratio is not a good way to measure performance. Here we’re just highlighting the fact that the most important aspect of HFA is more BBs and fewer Ks (for the offense).

    “Ks and BBs should always be treated as separate performance indicators and measured separately”

    Not necessarily. Most of the flaws of K/BB are solved by looking at K-BB. The important relationship is the difference, not the ratio. (K-BB)/PA is an extremely good measure of pitcher performance. (Personally I prefer to express it as (K-BB)/G by multiplying by 40 — scale is more intuitive.)

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    • Fresh Hops says:

      “(Personally I prefer to express it as (K-BB)/G by multiplying by 40 — scale is more intuitive.)”

      On a related note, I wish everyone would go the THT system for measuring K and BB rates, which is K/PA and BB/PA times the league average number of batters in each game, which gives a number similar to K/9 and BB/9 but doesn’t have the flaws of the per inning system (most notably, outs and hence innings are a function of fielding defense as well as pitching and so aren’t really DIPS.)

      Personally, if I’m evaluating a pitcher, I use FIP, tRA, xFIP or something and avoid evaulating without considering the third true outcome; it’s awfully important and the info is so available it’s almost fallacious not to look at it.

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  20. Jeff says:

    My guess my be travel effects – Away team could be more worn out from travel. Could look to see what the Bapip changes from the 1st game of series to last game.

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  21. Scott says:

    I’ve always bought into Jeff’s idea – the effect of travel. Not only that but all of the other distractions of being on the road – a lot of new things to do; being away from the wives and girlfriends – you know. If the home field advantage in baseball were only due to baseball things (familiarity of the field, official scorers, batting background, etc.) then the explanation wouldn’t do anything to help explain why there are also home field advantages in basketball, football and hockey. The travel is really the only common element. The effect of travel will translate into baseball things – poorer fielding, worse batting eye, etc., but I think those things are the effect and not the cause.

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  22. Mike says:

    Dave are there any noticeable home/away gaps at different parks? E.g. the Boston pitchers at Fenway have a more significant BABIP reduction compared to their road starts?

    It would be interesting to see if certain teams (with particular rosters) had a greater home field advantage in BABIP than others.

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  23. KJOK says:

    Nice article. I had an article along the same theme here:

    http://seamheads.com/blog/2008/10/24/how-the-home-team-wins/

    and have a follow-up in the works, which will address some of the issues raised in comments above.

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  24. MattS says:

    http://www.thegoodphight.com/2008/7/3/564256/homefield-advantage

    Have a look at this– I did it last summer. I pointed out this and several other very distinct differences between BABIP– the ratios of triples:doubles is very very different at home and on the road. Everything is affected, even SB% and HBP are statistically significantly different at home/away.

    Certainly a large difference in HFA is the batter’s eye and the mound. K/BB ratios are very different and that seems to be a source of things. HFA is much larger for teams that play each other less– even controlling for distance traveled.

    I also did a study of which games of the series HFA was largest for. I thought it was the earliest game due to jetlag/travel days/etc., but it was actually the MIDDLE game that had the largest HFA. Have a look at that too if you’re interested:

    http://www.thegoodphight.com/2008/8/17/595739/home-team-dominance-the-mi

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  25. Joker says:

    For a discussion elsewhere, to calculate .302 and .295, did you simply average the 14 year-end results or did you average all batted balls over the 14 years?

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  26. AM says:

    @Scott–Are away ballplayers really away from their girlfriends? Maybe they aren’t, and their hitting goes down as a consequence.

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  27. Daniel Knil says:

    Sagt mal, was ich schon immer mal wissen wollte: Hei?t es der Blog oder das Blog? Ich bin echt *verwirrt*! Helft mir. LG

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