By Jeffrey Gleason
Nine weeks into the NFL season, no teams remain unbeaten. This could’ve actually been said after eight weeks, after seven weeks, and after six weeks as well. Week 5 was the last time an unbeaten team remained, when both the Cardinals and Bengals were sitting at 3-0.
However, after these same nine weeks, five teams remain unbeaten at home. The Patriots, Broncos, Eagles, Packers, and Cardinals have yet to lose on their own turf.
Home field advantage is a phenomenon that gets a lot of traction in sports. Experts often use it to justify their predictions and betting lines usually reflect the perceived advantage of the home side. However, people often generalize home field advantage with a “one size fits all” approach, acknowledging its presence, but assuming it displays a constant impact across different situations.
With five unbeaten NFL home teams and the recent impetus of a road team finally winning Game 7 of the World Series (the Giants topped the Royals on October 29th to capture their third championship in five years), I was interested in how home field advantage was quantitatively different in different situations. How does it vary across sports? Do both good teams and bad teams experience the same advantage? Is it magnified in the postseason? What about differences in earlier eras? These are the questions I set out to resolve.
Although the why and when get more complex, the answer to whether home teams have an advantage in sports is actually what I believe to be a pretty straightforward analysis. To begin, I collected a sample of the home winning percentage for each team in each of the last ten years for the five US professional sports leagues (MLB, NBA, NFL, NHL, MLS). Although this sample isn’t random, the goal here is to look at home field advantage in a contemporary context and therefore it is sufficient. I compared these winning percentages to an expected winning percentage of .500 (in the aggregate, each sport necessarily has the same number of wins and losses). Here are the results:
*Data from sports-reference.com, oddsportal.com, and espnfc.com
**Data over the last nine years for NHL because of lockout and changed rules regarding ties prior to 05-06 season
***MLS winning percentage excludes draws
****The number of individual team seasons is equivalent to the sample size, as each individual team season produces a specific home winning percentage. Individual team seasons is the number of teams per league multiplied by the number of years of data taken
No matter what you believe drives the advantage, there is no disputing that teams in the aggregate win a higher proportion of their games at home than the expected .500. This advantage is also apparent across each of the five major sports, especially in the MLS, where home teams win over two-thirds of their decisions.
My next goal was to assess whether good teams and bad teams both enjoy the same advantage on their home field. Do only good teams play better at home or do bad teams also experience this situational edge? To assess this, I sorted each sport’s sample of seasons by overall winning percentage and then split each sample into a top half of “good” teams and a bottom half of “bad” teams. Although the top half would present a higher home winning percentage because of greater team strength, I used their mean overall winning percentage as the parameter of comparison to account for this difference. Finally, to assess which group presented a stronger home field advantage, if they both actually presented one, I took the difference between each group’s mean home winning percentage and mean overall winning percentage and used a two-sample t-test to assess whether those two were significantly different. Here are the results:
Both top-half “good” teams and bottom-half “bad” teams present a distinct home field advantage. Furthermore, despite a larger difference between home winning percentage and expected winning percentage for most top-half groups, none of these differences proved to be statistically significant. Therefore, we can conclude that home field advantage carries over across different strength teams, but isn’t more powerful for either group.
My next goal was to see how home field advantage presented itself in postseason play. The approach was identical to the one I used for the regular season data, but instead of just testing to see whether there was a home field advantage in the postseason, I also assessed whether that advantage was magnified compared to the regular season. There is an important distinction to be made about the structure of postseason play. Unlike the regular season, where home and road games are evenly split, each major sport (except the MLS which employs a two-game, home, away, aggregate scoring format and the MLB World Series where home field is determined by its All Star Game) gives the team with the better record more home games in a playoff series. This is clearly magnified in the NFL where a playoff round is just one game. Therefore it’s reasonable to expect that the home winning percentages during the playoffs (expect for in the MLS) will be higher because the better team is playing a greater proportion of home games. Results are below:
*Data from sports-reference.com, wikipedia.org, mlssoccer.com
**For MLS, Number of Playoff Games = those ending in a Win or Loss (ties are possible because of the two-game aggregate scoring format)
***postseason sample size = one whole postseason, not individual team postseasons
This is the first instance where the results aren’t consistent across all five sports. The NBA appears to provide a home playoff advantage that is significantly greater than the regular season home court advantage. We can’t just attribute this to the fact that better teams get more home playoff games, because this same scenario presents itself in both the NHL and MLB with no discernable extra effect on home playoff advantage. The NBA is more top-heavy (more standard deviation in teams’ winning percentages) than either the NHL or MLB, but regardless, the better team will still only get one extra, and sometimes zero extra, home games per round. However it does seem like an NBA team vying for a championship shouldn’t discount the edge that comes with the top seed and the top overall record in the playoffs.
Finally, the MLB home winning percentage across 168 individual team seasons and 25,890 games from 1953-1962 is 0.5404, almost identical to the current ten-year home winning percentage of 0.5422 (data from baseball-reference). This demonstrates that there is practically an identical advantage across these two different eras, and although we must be careful about drawing conclusions from one ten year period in one individual sport, this does point to the possibility that home field advantage, qualified by sport, is consistent over time.
In conclusion, home field advantage is certainly a statistically significant phenomenon. However, it gets much more interesting when you start to look at its intricacies. The MLS presents a home field advantage 7% higher than any other league and home field advantage is 5% more significant during the NBA playoffs than during the regular season. However, there is also startling consistency to many aspects of home field advantage. It is consistent across teams of different strength (and not more powerful for either group), throughout postseason play, and possibly across different eras. I think it’s safe to say home field advantage is anything but “one size fits all.”