If you ask an NFL bettor what home-field advantage is worth in terms of the point spread, there’s a good chance that the answer will be three points. And while that might have been true at one point in league history, it simply isn’t the case anymore.
Home-field advantage at a glance
• The 2020 COVID and fan-less season saw home- team performance hit all-time lows.
• Similar home-team performance in 2019 brings into question how much the return of fans will impact games.
• Home-field advantage has been diminishing over time, especially since 2007.
• Recent sharper declines since 2015 may provide insight into what we can expect in 2021.
The biggest story of the 2020-21 NFL season was how COVID-19 would leave its mark. The lack of fans was the most significant change to the game. Home teams had a record of 134-131-1 (50.6 percent), the lowest home win percentage in NFL history.
On top of that, home teams averaged fewer total yards than road teams for the first time since 1987. They also had an average scoring margin of +0.3, which marked just the third time home teams have had an average margin below a point.
NFL home lines on the decline
So we should expect home-team performance to drastically improve in the 2021 season with the return of fans, right? Maybe, maybe not.
In 2019, home teams went 136-124-1 (52.3 percent), the second-lowest home-team win percentage ever. On top of that, their scoring margin was just +0.1, the lowest in NFL history. Home teams also went 109-144-8 (43.1 percent) against the spread*, further signaling that NFL home-field advantage may be dwindling quicker than the market has realized.
*Includes playoffs but excludes the three 49ers games that were played in AZ due to the city of San Francisco’s ban on contact sports.
And make no mistake, bookmakers are aware of the apparent marginalization of home-field advantage. In 2020 the average home line reached an all-time low of -1.3, breaking the previous low mark of -2.0 from 2019. Not only do 2019 and 2020 represent the two largest decreases in average NFL home lines in the last 20 years, but they also represent the only instance during that span in which there were decreases in average home lines in back-to-back years.
In the 2007 season, a trend began in which the average home line was higher (or closer to zero) than the three-year moving average for home lines (example: if looking at 2007, the three-year moving average would include the 2004-2006 seasons).
When looking at the graph below, this is where the green line (representing average home line) crossed with the gray line (representing the three-year moving average) and began to pace above it.
What this demonstrates is the pace at which home lines are diminishing. In the 14 seasons since 2007, there have been ten instances where the average home line has been higher than the three-year moving average. In three other instances, the average home line was either equal to the three-year moving average (2011) or was lower by a mere tenth of a point or less (2013 and 2014).
In 2018 the average home line was 0.27 lower than the three-year moving average, and home teams ended up going 48.4 percent against the spread despite a 60.2 percent record straight up.
Factors in NFL home-field advantage
So why was home-field advantage in the NFL decreasing before the 2020 season when fans were largely absent?
One significant factor is the increasing physical and mental ease of team travel. NFL teams employ sleep scientists for rest logistics, and new technology allows teams to continue game preparation more efficiently while traveling.
But to suggest fans have no impact on home-field advantage would be asinine. Studies across multiple sports have shown that the presence of fans has influenced the rate at which officials will make favorable calls towards the home team. This bias is observable in the 1999 NFL season, in which the league introduced challenges and instant replay.
Before 1999, home teams had a 62.0 percent win rate in the three seasons and a 52.6 percent cover rate. In the three seasons following the change, home teams had a 56.9 percent win rate and covered 49.7 percent of the time. This demonstrates the natural bias officials have towards home teams, as before 1999, they went largely unchecked on calls that were influenced by fan presence and pressure.
If we examine the 2020 season (which is admittedly a tiny sample), there is data to suggest that the presence of fans affects home-team success. In a study done by Ben Caterine of the Northwestern Sports Analytics Group, home teams won 54 percent of games that allowed any number of fans compared to 47 percent of games that allowed no fans. However, the study also found no meaningful correlation between higher attendance and home-team performance.
So what kind of home-field advantage number should we expect in 2021?
NFL home-field advantage in 2021
It would be irresponsible to ignore home-team performance in the past two seasons entirely. However, it is equally irresponsible to look merely at 2019 and 2020 and assume that home-field advantage is now simply gone and is the new norm. The answer is very likely somewhere in the middle.
If we return to the previously mentioned trend that dates back to 2007 (and exclude the 2020 fan-less season), the average year-over-year decrease in the average home line has been 0.1. If we apply that to the 2019 season (the last season with fans) and its average home line of -2.0, we get 1.9 (remember that number).
We know the diminishing of NFL home-field advantage has accelerated as of late, so what if we close the window a little?
When looking at just the last five seasons with fans (2015-2019), the average home line during the regular season has been -2.1, and the average home winning margin has been 1.7. It just so happens that splitting that difference between the two gives us the same number of 1.9 from before.
|Year||Average home line||Delta vs. PY||3-year rolling average||Line vs. rolling average|
Of course, that is merely an estimated average, and it is far from a number you can reliably apply broadly to every game of the 2021 season.
The magnitude of home-field advantage for any game can range widely depending on the actual home team and location, travel distances and logistics for the road team, rest advantages, disadvantages, and more.