Posted by Jason Lisk on January 7, 2008
There seems to be a generally held belief that teams with byes in the first round of the playoffs have an increased advantage in the semifinals. For example, since 1990, the home teams in the semifinals have won 77.9% of games, compared to 69.1% in the wildcard round, and 58.8% in the championship games. Not only has the winning percentage been higher, there have been more noticeable blowouts in the semifinals than any other round.
The problem with these numbers is that they do not control for matchup. To (try to) answer the question of whether the bye week increases home field advantage, I will use the regular season SRS ratings going back to the AFL-NFL merger, and look at the average expected results, based on regular season ratings, and average actual results in the playoffs for the home team. (I did exclude the two strike seasons of 1982 and 1987, but included every other year from 1970-2006).
The difference between the average actual result for any category, and the average expected result, if the game was played at a neutral setting, is what I will refer to as "home field advantage".
I divided the matchups into four categories: the wildcard round matchups; the semifinal matchups where teams were on equal rest, which were all semifinals from 1970-1977, and the semifinals between two division winners from 1978-1989; the semifinal matchups where the home team had extra rest, which were the matchup involving the wildcard winner from 1978-1989, and all semifinals since 1990; and the championship games.
Just looking at the four categories, the semifinals have the largest home field advantage, when compared to both the wildcard round and the championship game round. However, the semifinals where home teams had extra rest (HFA = +5.9) show no difference than those where the home team had no extra rest (HFA = +6.0).
However, these numbers are not controlled for climate and types of matchups. For example, until 1990, the NFL had a rule that divisional opponents could not meet in the semifinals. The home field advantage in games involving two divisional opponents is lower (by more than 2 points) than games involving conference opponents. When I remove the divisional matchups, and look only at playoff games in each round involving only conference opponents, here are the results:
type no. home away diff. PF PA PF-PA HFA =========================================================================== wildcard 59 4.6 3.2 +1.4 24.2 16.9 +7.3 +5.9 semis-no rest 52 7.3 4.9 +2.4 25.1 16.7 +8.4 +6.0 semis-extra rest 77 7.5 3.4 +4.1 26.2 16.5 +9.7 +5.6 champ games 57 8.3 5.9 +2.4 22.7 17.5 +5.2 +2.8 ===========================================================================
When divisional games are removed, the home field advantage in the wildcard round, the semifinals when home teams had no extra rest, and semifinals with home team having extra rest, are all very similar. And much higher than the average home field advantage in the regular season. But you can see why the perception is that there is an increased advantage with extra rest. The average margin in these games has been +9.7 for the home team. But the average difference between teams is also higher, because in virtually every matchup, the home team in the semifinals is either better, or at least roughly equal to the opponent. In the wildcard round (and for that matter, in the championship game round), there have been a fair number of matchups where the home team is seen as weaker by SRS.
The reason the advantage is closer to 5.5 to 6.0 points in the playoffs is probably more due to climate effects in January. Here, I borrowed from data compiled by frequent commenter Brian Burke, who posted this study on his website. In the first guest post I wrote for this blog last October, I had looked at climate and home field advantage and found median daily high temperatures for the month of December. But Brian went a step further recently and pulled wind chill data. After having my nose nearly freeze off the other day when I filled up the car, while the thermometer said 25 degrees but the wind was whipping, I decided I liked Brian's more detailed data. The same temperature in Chicago or Kansas City may feel differently than in Seattle, thanks to wind speeds. The information is for the month of December, but I think using the December numbers for each city is going to give us a pretty good idea of temperature differences between cities during playoff time in January as well. So, rather than re-invent the wheel, I used Brian's wind chill temperatures for December for each city. There were a few that I had to guesstimate on my own because the team had re-located or moved to a dome, but played in the playoffs as an outdoor team in the past, such as Atlanta, Saint Louis, Minnesota and Detroit.
Here are the Home Field Advantages for all playoff games, sorted by difference between the home team's December wind chill temperature, and the road team's December wind chill temperature. The positive numbers mean the home team was warmer than the road team, and negative numbers mean the home team was colder than the road team.
temp diff. no. home away diff. PF PA PF-PA HFA =========================================================================== +35.1 or more 29 6.6 3.1 +3.5 24.8 20.3 +4.5 +1.0 +15.1 to +35.0 57 6.6 4.6 +2.0 24.3 15.6 +8.7 +6.7 within 15.0 deg. 119 6.7 4.3 +2.4 23.5 18.4 +5.1 +2.7 -15.1 to -35.0 46 7.4 5.0 +2.4 23.8 14.6 +9.2 +6.8 -35.1 or more 47 5.7 2.8 +2.9 27.1 16.0 +11.1 +8.2 ===========================================================================
When the teams are relatively similar climate-wise (within 15 degrees on average), the playoff home field advantage is a moderate +2.7 points. But when the home team has a cold weather advantage, the home field advantage increases dramatically. When the home team is -15.1 to -35.0 degrees colder than the visitor, the HFA in the playoffs is almost a touchdown. When the home team is more than 35 degrees colder than what the visitor is accustomed to, the advantage is over 8 points on average.
So, in review, home field advantage is significantly increased in the playoffs versus the regular season, primarily due to cold weather effects. Thus, the home field advantage in the semifinal round is higher than home field advantage in the regular season. However, it is not any higher than either the semifinals where both teams had the same amount of time to prepare, or the wildcard round games, once we account for type of opponent and climate. This suggests that the bye, in and of itself, does not increase home field advantage for the home team. The observed increase in winning percentage and average margin in this round is due to the greater difference between the home team and road team in average ability.
While most people assume that it is in the semifinal round that the home field advantage is different, it is actually the conference championship games where home field needs to be adjusted relative to the rest of the playoff games. Why is this? My speculation is that it may be partially due to the types of road teams that are playing in the championship game. As a general rule, these are really good teams that are, again, generally very good at pass efficiency. Earlier this year, I observed that, in the regular season, for average or near-average NFL teams, those teams that were good passing, poor run stopping teams tended to show a smaller home/road split in performance when compared to poor passing/good run stopping teams. If pass efficiency decreases the opponent's home field advantage, then these road teams in conference championship games may just be much better on the road (relative to playing at home) than the average NFL team.
There is one exception to this rule about conference championship games showing almost no home field advantage, and again, it is related to climate. In the thirteen conference championship games where the home team's venue is more than 35 degrees colder than the visitor's, the average result has been 29.5 points for the home team, only 9.9 points for the visitor, for an average margin of victory of +19.6 for the home team. Using the teams' SRS rating, the home teams were expected to win by +5.0 points, which means the home field advantage in these games has been over two touchdowns. In the remaining 57 championship games where the visitor is not at an extreme cold climate disadvantage, the home field advantage has been virtually non-existent, at +0.6 points.