I had a theory that part of home field advantage may come from fumble recovery rates. Specifically, I was thinking of the kind of fumble that results in a pile of humanity fighting for the ball by doing things to each other only elsewhere done in prisons. It seems that the officials often have no better way of determining possession than by guessing which player has more control of the ball than the other guy. Sometimes it seems like they have a system--pulling the players off the pile one by one until they can see the ball. But in the end, they're still relying on their own judgment. There are complicating factors. Where was the ball when the play was whistled dead? When was the original ball carrier down? Was it a fumble or incomplete pass? In many cases, the process is analogous to basketball referees determining possession of a "jump ball" by their judgment of which player has better grip, or which player ultimately ripped the ball loose.
Perhaps the influence of the crowd had an effect on the officials by biasing their judgment. It's plausible because their have been many academic studies documenting the psychological effect of a home crowd on officiating in several sports. Much of the research focuses on penalties and fouls called by the officials, but what about other matters of judgment? Fumble recoveries might shed some light.
If the fumble recovery rate of home teams is significantly greater than away teams, then we'd have evidence that NFL officials are favoring home teams. The table below lists home and visiting team's fumbles and fumbles lost from the entire 2007 regular season encompassing 256 games.
It appears that although visiting teams fumbled slightly more often, they lost possession less frequently. Neither difference is statistically significant, however, indicating that officials are unbiased in that department.
Although my fumble theory was a bust, what about penalties. Could the difference in penalties given to home and away teams be large enough to explain most of the home field advantage in the NFL? But if visiting teams in fact penalized more, it wouldn't necessarily indicate officiating bias. It could be due to crowd noise or other factors.
The table below lists The visitor and home penalty and penalty yard averages for the 2006 regular season.
I was very surprised by how small the difference is. On average, visiting teams only have 0.4 more penalties called (and accepted) on them than home teams for a difference of only 2 yards. I would expect the difference to be greater because of false start and delay of game penalties due to crowd noise.
In 2006, home teams won 55.6% of regular season games. According to the in-game model at Football Prediction Network, the difference of 2 penalty yards can only account for about 0.9% of the 5.6% home field advantage.
It appears that neither fumble recoveries nor penalties account for much of home field advantage in the NFL. Other factors such as travel fatigue or motivation are likely to be much more important. So I came up empty handed in the research...or so I thought until I came across some gems at Referee Chat Blog when doing some background research.
The author tracks officiating data from week to week, crew by crew. One of the most interesting things he's found is that crews don't tend to consistently favor home teams more than visiting teams across seasons (correlation = -0.04). Contrary to what was found in the study of officiating in British Premier League soccer I linked to above, NFL officials do not indicate a susceptibility to home crowd influence.
Many of the author's conclusions are based on differences in very small sample sizes (and he seems to realize this), but the data there are sound. Rex definitely knows his refs.