The final weekend of the NFL regular season is upon us, and my favourite Carolina Panthers will play for the NFC South division title against the Tampa Bay Buccaneers tomorrow. The winner wins the division and goes to the playoffs. The loser goes home disappointed [*]. The game is being played in Tampa, which conveys some home advantage to the Bucs. How much home advantage, and why?
Those are essentially the questions answered by this new article by Adam Cook (State University of New York at Fredonia), published in the Journal of Sports Economics (sorry, I don't see an ungated version online). There have been lots of studies of home advantage across many sports, including the NFL. The problem is that, while it is obvious that there is home advantage (home teams do win more often), it isn't clear why home advantage exists. Cook notes that:
Various mechanisms have been proposed to explain the persistent advantage enjoyed by the home team: direct crowd effects, home crowd influence upon referee decisions, travel hardships for the visiting athletes, unexpected temperature, wind and precipitation shocks may all explain portions of the persistent difference in success between home and visiting competitors.
Cook focuses his attention on the home crowd. However, untangling the effect of attendance on home advantage is difficult, because:
...better home team performance will positively affect demand for stadium attendance, but greater stadium attendance may positively affect home team success at the same time; the two quantities are likely simultaneously determined...
To overcome this problem, Cook leverages the COVID public health mandates, which limited some stadiums to 31,700 fans, while others had zero fans, during the 2020 NFL season [**]. The good thing about this approach is that COVID mandates created sharp constraints on attendance that weren’t driven by team quality or local demand. However, COVID mandates were not the only disruption in 2020, and if any of those other disruptions affected game outcomes directly, the instrument could partly pick those up as well. That said, it’s an plausible instrument and one that that others have used (although in less comprehensive analyses than Cook's).
Cook uses data from the 2009 to 2022 seasons, and uses a binary variable for the 2020 season as an instrument for stadium attendance (and, in separate analyses, as an instrument for how full the stadium was, in terms of percentage of capacity). Cook looks at the impact on a number of variables, including the probability of a home win, home advantage (measured in points differential), total points scored (by the home team, the away team, and both combined), and various measures of penalties (to pick up differences in referee decisions). To deal with travel hardships, Cook includes travel distance (and number of time zones crossed) in the model, while to deal with the weather variables, he initially includes temperature, weather, and precipitation as variables in the model, then estimates models separately for games played indoors (where weather cannot be a factor) and outdoors (where it can).
Cook finds initially that:
The stadium attendance effect on home winning percentage, home field advantage, total points scored and visiting team points scored are significant at the 5% level...
...for every 10000 fans who attend an NFL game, the probability of a home team victory rises by 1.10% and the home field advantage grows by 0.3323 points. When evaluated at the average NFL stadium attendance, 63407 fans, home attendance accounts for 2.11 points (or 97%) of the mean 2.17 point home field advantage observed in the full sample.
The total number of points scored falls by 0.6542 per 10000 fans, but the reduction in aggregate scoring is not shared between home and away teams– instead it is the visiting team who suffers more, scoring 0.4933 fewer points per 10000 fans.
Summing up, home advantage is related to crowd size, and operates primarily through the away team scoring fewer points. Turning to the effect on penalties (and thereby, influence on refereeing crews), Cook finds that:
For every additional 10000 fans in attendance, the total number of penalties rises by 0.2499 total penalties per game. This increase in total penalties is shared equally between the home and visiting teams, however, with home teams receiving 0.1077 extra accepted penalties and visitors an extra 0.1052 extra accepted penalties per 10000 fans in attendance. The effect of attendance on penalty yardage is also comparable, with home teams receiving 0.7461 extra penalty yards and visitors receiving 0.7754 extra yards per 10000 fans in attendance.
There is no home advantage in terms of penalties, so NFL refereeing crews do not appear to be biased towards the home team. At least, not through a mechanism of crowd influence on penalties.
What about travel effects? In an earlier OLS regression, Cook finds that the correlation between the distance the visiting team travelled and game outcomes, and the correlation between the number of west-to-east time zone changes the visiting team experienced and game outcomes, are both small and statistically insignificant.
Turning to weather, Cook's separate analyses between games played indoors, and those played outdoors, reveal that:
When compared to the full sample results, rising attendance in outdoor games no longer has any measurable effect on the probability of the home team winning, nor on home team points scored and the effects on home field point advantage, total points scored and visiting team points scored are all diminished compared to the full sample estimates...
...playing indoors is associated with a larger home field advantage– a much larger advantage. For every 10000 fans in attendance at an indoor game, the probability of a home team win rises by 3.15% and home advantage rises by 0.6227 points– almost double the effect found using the full sample and 243% larger than the attendance effect on home advantage at outdoor games. Evaluated at the mean indoor attendance, 64678 fans, the average home field advantage rises to 4.03 points, or 186% of the average home field advantage observed in the full data sample.
Total points scored falls by 1.412 points, and this decrease is accounted for by a 0.3947 point decrease in home team scoring, but a 1.017 point decrease in away team scoring per 10000 in attendance, suggesting that greater indoor crowd size negatively affects both teams’ scoring output compared to the full sample and outdoor sample results, but affects the visiting side to a much greater degree.
Penalties were similar between indoor and outdoor games. What we learn from these results is that the home advantage is not driven by the weather, because it is bigger when weather is not a factor (in indoor stadiums). So, going back to the list of explanations for home advantage that Cook begins with, he has eliminated home crowd influence upon referee decisions (no differences between home and away teams), travel hardships for the visiting athletes (not statistically significant), unexpected temperature, wind and precipitation shocks (the home advantage is bigger when games are played indoors). That only leaves direct crowd effects, unless we are missing something. Cook concludes that:
In the absence of any detectable NFL referee bias, these results suggest that it is the NFL home stadium crowd itself that is directly affecting the on-field performance of the home and visiting athletes. Despite a lack of data tracking in-match noise intensity, given the acoustic differences between indoor and outdoor stadiums, this effect is likely related to crowd noise levels.
That conclusion will certainly please many fans attending NFL games, who really believe that they have a direct impact on team performance. The 12th Man is real!
Should my Panthers be worried? Based on these results, they should be somewhat worried about the crowd, but not as much as for some other opponents. Raymond James Stadium is outdoors, and although Cook found no significant effect on the probability of the home team winning, there was still an (albeit smaller) effect on home advantage measured in points. The betting odds have the Panthers as 2.5-point underdogs. With a 70,000-capacity stadium, Cook's results for outdoor games imply that 1.8 points [***] of that spread comes from the home advantage.
Let's go Panthers! Keep pounding!
*****
[*] Although, if the Bucs beat the Panthers and then the Atlanta Falcons beat the New Orleans Saints the next day, the Bucs, Panthers, and Falcons will all finish with a record of 8-9. Due to round-robin results between those three teams, the Panthers win the division. Hopefully, a Falcons win won't be necessary!
[**] It was eerie to watch that season, with 'simulated' crowd noise for the stadiums that had no fans present.
[***] The coefficient of 0.2563 points differential is for each 10,000-person increase in attendance. The difference between 0 and 70,000 attendance is 7 times the coefficient, or 1.7941.
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