How much of home field advantage is the officiating?

There are many factors that could cause home field advantage (HFA): refereeing, travel, park familiarity, crowd support, and many more. And there have been studies that have checked some of these. In “The Diamond Appraised,” for instance, Craig Wright found no effect for crowd size, and that teams playing in brand new parks had only slightly below-average HFAs. But, when the new park was in the same home city, the HFA was actually higher than average – it was brought down by lower-than-average HFAs for the new-city parks.

Wright also found a huge HFA for triples – he advanced the theory that players at home know exactly how to play balls hit off the wall to limit opposing hitters to two bases.

I just ran across this recent NBA study on home field advantage by Roland Beech. What that study suggests to me is that refereeing plays a significant role.

The study compared NBA home and road statistics in several different categories. The home team was superior in every category – with one exception: free throw percentage. Here are some of the results:

............ Home ... Road
--------------------------
FG% ........ .460 ... .447
Off. Reb. %. 30.6 ... 28.5
Def. Reb. %. 71.5 ... 69.4
FTA ........ 27.0 ... 25.6
FT% ........ .746 ... .744


Where does the refereeing come in? Well, free throw percentage is the one category in basketball where there’s no possible influence from the referees. And in that one category, home teams had almost exactly the same performance as road teams.

That’s not what I expected to see. I would have expected a modest improvement in every category across the board. If players are just physically and mentally not at their prime when on the road, that should affect free throws too. But there was hardly any difference there.

With regard to foul calling, Beech notes that the the free-throw HFA accounted for 1.3 points per game. The overall HFA was 3.4 points per game. Still, that’s pretty significant, 38% of the total. You’d have to acknowledge, though, that not all of that difference – in fact, not any of it -- need be referee bias. It could just be that road players commit more infractions than home players. But still, the difference is suggestive.

And all the other categories could also be significantly influenced by referees. If road players know they are more likely to be called for a foul, they will play less agressively. And so even in cases where a foul isn’t called, their stats should be lower, both on offense and defense.

Similar research for baseball can be found in this study from Tom Meagher (previously mentioned in this blog in the comments here). Meagher found that most of the HFA can be explained by walks and strikeouts. In fact, if home and road teams were equal in every category except K and BB, the home team would have a winning percentage of about .520. Since the actual HFA is only about .532, we can see that balls and strikes form a very large proportion of total home field advantage. And, of course, balls and strikes are the category in which umpire judgement figures most prominently.

And, as in basketball, the other categories would also be influenced by umpires. A pitcher on the road who notices a shrunken strike zone will get behind in the count and have to give batters better pitches to hit. This could account for the fact that there is still some HFA in the results of ground balls put into play – the umpire effect would mean home batters hit “better” ground balls off worse pitches.

Following this line of thinking, it’s even theoretically possible that *all* of the home field advantage could be attributed to refereeing. I don’t think that’s the case – I believe that other factors exist (Wright’s explanation of the triples effect, for instance, seems right to me). But I do wonder if refereeing is a bigger effect than originally thought.

How would you find out? I’m not sure you can. But you can start by finding categories where, as for free-throw percentage, refereeing should have little or no effect. You could look at NFL field goal conversions, or the results of NHL shootouts. If every such category shows no advantage to the home team, that would reinforce the theory that HFA is predominantly refereeing.

And you can also look at sports where refereeing isn’t a factor at all – golf or tennis, say, or bowling. The HFA in those sports could give a strong suggestion of how much of the advantage in other sports is inherent in the players’ performances. Anything above that amount, in refereed sports, we’d suspect is a function of the officiating.