NBA Home Court Advantage: Really Just The Refs?

posted in NBA

Today at the 2011 Sloan Sports Analytics Conference, Scorecasting author Tobias Moskowitz gave a great talk on home field advantage across various sports, and how he believes it can largely be explained by referee bias. We’ll go over his main points, then present some interesting numbers that we feel aren’t entirely explained by his claim.

Some of the evidence he presented demonstrated that home and away teams perform the same in situations where there is little referee interference, and that the home advantage is not travel related:

  • NBA FT shooting — no difference between home and away FT% (actually, according to our numbers below, there is a small difference, but it amounts to only a 0.05 point per game advantage)
  • Shootouts in NHL — no difference between home and away success rate
  • Pitchers/batters in MLB — for things like the percent of pitches in strike zone, mean velocity, horizontal movement, swing probability, and contact rate, there are no differences between home and away
  • Travel fatigue — there is no real difference in winning percentage based on distance traveled

He also presented evidence that referee bias in favor of the home team is widespread and significant:

  • In soccer, there is around twice as much injury time when home team is down by one, versus when the home team is up by one.
  • In baseball, there is no advantage for the home team in non-crucial situations, but a 0.5% difference in ball-strike calls in “very crucial” situations.
  • Over a season, visitors get 516 more strikeouts and 195 fewer walks than they “should.”
  • In the NFL, more penalties are called on visitors, and instant replay decisions tend to favor of the home team.
  • In the NBA, fouls and other violations are called more often on the road team, and the home team gets the benefit of the doubt in situations where there could be either an offensive or defensive infraction called.
  • The crowd seems to affect the referees. The larger and more vocal the crowd, the bigger the influence on the measured statistics. In other words, home advantage grows with attendance.

Overall, he painted a fairly complete picture: clearly referees are a part of home court advantage. But, are they truly responsible for the majority of the advantage?

We dug into our NBA statistics archives (going back to the 2003-2004 season) and pulled out a variety of home versus away stat splits. There should be no need to adjust for quality of play, as every team plays an equal number of home and road games. Below the table, we’ll highlight a couple of things that make us wonder if referee bias really tells the whole story:

Off Rebounds11.0711.520.454.0%
Def Rebounds29.7030.751.053.5%
Total Rebounds40.7742.271.503.6%
Off Reb%26.5%28.0%1.5%5.4%
Fastbreak Points10.4711.891.4312.7%

First, check out the steals and rebounds. The home team registers more of each, and their advantage is larger than the fouls advantage: +0.12 in steals, +0.45 in offensive rebounds, and +1.05 in defensive rebounds, compared to -1.02 in fouls. Even if every single extra visitor foul is called on what would have been a legitimate rebound, home teams still have an advantage in steals and rebounding. Beyond that, the foul disparity also needs to account for the shooting percentage difference, which it can’t do if we assume all the extra fouls occur in rebounding situations. There is just not enough of a foul difference to account for the disparities in the other stats.

Second, examine the fast break points. The home team has a 1.43 edge, despite only 0.12 extra steals. This seems like it must be due to some kind of qualitative difference in the way the two teams are playing, because the extra opportunities themselves don’t seem to be enough to account for it. Unless, perhaps, that extra foul occurs on a fast break. But, again, that leaves us with nothing to explain the other home/away differences.

Third, look at the shooting percentages: the home team is +0.2% in FT%, +0.5% in 3pFG%, and +1.4% in 2PFG%. As above, let’s assume that the extra foul on the visitors always occurs on what would have otherwise been a legitimate field goal defensed. So, add 1.02 misses to the home team’s line. If we add all those misses to the 2PFG% calculation, the home team still ends up at 47.9%, or +0.7% over the away team. Again, the fouls themselves aren’t enough to account for the disparity. Surely we’ve all had the experience of playing for the first time on an unfamiliar court, and feeling that we can’t quite judge our shots correctly. It’s hard to imagine this doesn’t happen to some small extent in higher levels of sport, even if on a much smaller scale.

Moskowitz clearly showed that referee bias plays a large role in home court advantage. All we’re asking is: are we sure it’s the entire story? After all, if referee performance is affected drastically by the home crowd, why wouldn’t player performance be as well?

  • Jeff Anderton

    amazing article – i wish i had more to add but i am not that smart – one thing though, maybe players become aware of these referee tendencies, so they dont give as much effort, maybe from a subconcious stand point, because they expect more calls to go there way, and road teams dont expect to get more calls to go there way – so because the home team doesnt give as much effort and the road team gives more effort, both because of the perceived referee biases, that the homecourt edge is nullified…

  • David Hess

    Jeff, that’s a good point, that the effort or strategy of both teams may change due to the referee feedback. So perhaps some of the advantages in the above stats come from visiting players playing less aggressively. However, then we have to ask: should the credit for that less aggressive play really fall entirely to the referee feedback, or would some of it be due to the psychological effect of “entering the lion’s den?”

  • Jeff Anderton

    actually, I was saying that the home team doesnt have as much of an advantage as it seems because of the ref bias – i was saying that the home team might slack off, and the road team might play harder, but the ref bias still gives an edge to the home team, but the edge doesnt really “show up” in the final stats becaue of the home and road team efforts negating each other – does that make sense?

    I thought the article was wondering why the home team doesnt have more of a long term edge, because the refs give the home team more calls – but i was saying that the final numbers equal out…

  • David Hess

    Sorry, looks like we both reversed each other. The article was saying there IS a home field advantage that “shows up” in the final stats, and it seems to be larger than what the foul differential can account for. Your idea makes sense, if you apply it in the reverse direction from what you intended. The visitors might be less aggressive becuase they don’t want to get called for more fouls, which is what allows the home team to gain an edge in so many categories.

  • Ricardoathome

    why are the win loss records always in favor of games played at home.

  • Anonymous

    Ricardo – what we show above, which is nothing new, is that the home team has an advantage in most recorded statistics. Those advantages add up to an overall advantage that’s reflected in win-loss record.

    Neither us nor the Scorecasting author is arguing that home field advantage doesn’t exist. The issue is that he claims the entire advantage comes down to referee bias, while we think there might be more to it that that.

  • tonyz

    The numbers don’t necessarily prove that referees are biased.  As many of the numbers show, teams perform worse on the road.  Perhaps the increased number of fouls charged against the visitors is simply due to their sloppy road play.

  • David Hess

    Thanks Tonyz, that’s similar to what we’re trying to say. While we do believe that at least SOME of the home advantage is due to refereeing (the soccer extra time issue and the baseball umpiring that we mentioned above are good examples), we also think that it may not be the whole explanation. As you said, some of the differences between home and road performance fouls could be due to the home/road teams playing different styles.

  • Samson

    When things are going a teams way, that adds to their motivation to play better and makes them want to increase the pressure and maintain said level of pressure to “put salt in the wound” and limit the away teams momentum as much as possible. I believe home referee bias, while limited to a few situations, sometimes more, plays a decent part game-changing situations. Baseball (home plate umpire, stolen bases and first base calls), Basketball (shooting fouls, blocking/charging calls), Football (pass interference mainly), Hockey seems to be a little more balanced but you still see the visiting team receiving a lot of wishy-washy calls that would ordinarily not be called, especially during playoffs this happens. In hockey the refs will usually let teams play rough during the first period, setting a precedent for the entire game, but if they are more strict in the 1st that usually means they are going to be strict the entire game, just that they will tend to call more penalties on the visitors. A lot of high stick calls get missed. Boarding calls are at the discretion of the referee. Deflections on icing are often missed by referees. Perhaps the refs want to keep games closer for sake of the fans and television ratings? Who knows. All these, though, seem to be calls that have clear ability to be called fairly and often do not. These statistics are interesting though, but I think when a team has things going their way they will generally play better and be more motivated to do well when they think they have refs on their side.

  • David Hess

    Samson – Good point. Even a small bit of ref bias, if applied in key situations, can greatly impact the outcome of the game. And, there’s definitely a mental aspect to a team’s performance that could be affected by the feeling that the refs are on their side. But that’s going to be extremely difficult to separate from the mental effect that being at home has. In other words, if we know that teams play better by X% in some category when at home, how much of that is caused by psychological effects of having the crowd behind you, and how much is cause by psychological (or strategic) effects of having the refs behind you.

  • dmc

    This comment is way late for this post, but just wanted to throw out that I think you need to double the effects of the foul differential.

    Assuming each team on average commits the same number of actions that could be considered fouls, a differential of one foul means two plays not one play. A foul called on the away team, and a foul NOT called on the home team.

    That doesn’t mean that fouls are 100% of the scoring differential, but I think they are twice as significant as you are describing.

  • David Hess

    dmc — I’m not sure exactly what you mean here, but I’m guessing you are referring to this section:

    “The home team registers more of each, and their advantage is larger than the fouls advantage: +0.12 in steals, +0.45 in offensive rebounds, and +1.05 in defensive rebounds, compared to -1.02 in fouls. Even if every single extra visitor foul is called on what would have been a legitimate rebound, home teams still have an advantage in steals and rebounding.”

    I don’t see what the error is here. The “foul NOT called on the home team” that you mention is what turns into an extra defensive rebound. Look at it this way: if there is a single rebounding opportunity at each end of the court, and the home team snags their defensive board, but the away team is whistled for a foul, you get -1 in fouls and +1 in boards for the home team. Change a single play (assume the away team doesn’t get whistled for a foul), and the teams are even.

  • yessah3

    This also doesn’t take into consideration the importance of the fouls. Were the fouls called when the team was on a run…late in the game when the score was close, on a key player, on a player who had a lot of fouls already, etc…. So it isn’t just the foul, it is the importance of the foul.


    Yep, good point.

  • Jack

    I read this after watching a NBA playoff game (Heat v Pacers, game 1 2014 conference finals) where the away team got called for 3X as many fouls. There’s already been comments on the psychological effects of poor/unbalanced officiating, but i think it’s worth chiming in. on it again:

    Statistics measure numbers and create correlations but we’re the ones who have to figure out why the numbers say what they say. I believe from experience playing, coaching, and watching (all subjective) that unbalanced officiating encourages one team to play more aggressively and another to play more passively. Especially in basketball, if one team is allowed to play more physically than the other, they’ll outperform the other team because they aren’t bound by the same rules. Unbalanced officiating does not allow for balanced play. Therefore, the effects of poor officiating cannot simply be measured on a 1 foul to 1 rebound scale. The effects are cumulative on performance.This is a legitimate possible reason for the correlation between playing at home and better statistical overall performance.

    In other words, I don’t see how displaying the unbalanced statistics provides us with any value in deciphering why home court advantage exists. It doesn’t in any way disprove the hypothesis that unbalanced officiating is the ENTIRE cause. It may just as well prove it as it does disprove it. If we are trying to say Tobias Moskowitz may not have described the entire reason for home court advantage, these statistics fail to show it.


    Taking a look at this again with fresh eyes (it’s been a couple years since I last viewed this post), I agree with you. It’s plausible that the above stat differences could be the result of a change in playing style/aggressiveness due to fouls being called differently.

  • yessah3

    I couldn’t have said it better myself. I would also add that it isn’t just the number of foul calls, but the importance of those calls. You could literally have a team with the same amount of calls but one team gets the calls late in the game or during key runs, etc…