Mythbusting the NFL Playoffs

It’s time for the NFL playoffs, and your Seattle Seahawks find themselves with the top seed in the NFC. The best point differential in the conference, in addition to a bye in the first round and home-field advantage  after that, makes Seattle a legitimate favorite to reach the Superbowl. I have now unveiled our mathematical playoff predictions based on a study of the past 14 seasons, but here I want to discuss some interesting findings on which I have based (and not based) this prediction model. Below you will find some common claims, followed by whether or not past data actually supports that claim.

Home-field advantage exists in the playoffs

You bet it does. In the past 140 (non-Superbowl) playoff games, home teams have outscored the visitors by an average of 5.1 points per game, but that statistic in and of itself doesn’t really support the statement. Remember, home teams are playing at home essentially because they are better. Is it talent or rabid fans that makes the difference? Controlling for each team’s ability using  regular season point differentials, we see that playing at home still provides a modest 3.8-point advantage (p-value = 0.004), or put another way, an increase from 50-percent chances of winning to 62 percent (p-value = 0.007).

Dome-field advantage exists in the playoffs

Does a team that hosts in its own dome enjoy an even greater home-field advantage? Sort of. This one is trickier to dissect, but here goes. Theory might suggest that a dome team is more accustomed to the noise levels, and is better built to win in a dome (read: is a passing team). Indeed, according to Advanced NFL Statistics, domed and roofed teams* did tend toward a more effective passing attack, but only slightly. However, while domes may be able to create certain advantages for their home teams, a road team that is used to playing on the slower natural turf—in various weather conditions—might feel a bit like John Carter on Mars when traveling to play in a dome. Let’s see what the data has to say. If you divide it all up, there are four distinct types of matchups that could occur. Below I have summarized the playoff data for each.

Home Away Games Pt. Diff. Error (5%)
Dome/Roof Open 24 +6.6 5.7
Open Dome/Roof 25 +8.5 6.4
Dome/Roof Dome/Roof 9 +16.8 9.0
Open Open 82 +2.4 3.2

The advantage that a home dome team enjoys when facing an opponent accustomed to the elements (6.6) is quite similar to the advantage that a home team accustomed to the elements enjoys when facing an opponent that is more comfortable in a dome (8.5). In other words, if there is a slight advantage to being a home-dome team, there may be a slightly bigger advantage to forcing the opponent to come play at your cold, open air stadium in the playoffs. In that respect, it may not be “dome-field advantage” so much as an “oh-shit-I’m-not-used-to-this advantage.” Then again, none of this is statistically significant, so maybe it’s nothing at all. That’s why I said “sort of” up there. *Teams that play in a dome, or under a retractable roof during bad weather, currently include Arizona, Atlanta, Dallas, Detroit, Houston, Indianapolis, Minnesota, New Orleans, and Saint Louis, according to Wikipedia

One-dimensional teams are screwed come playoff time

I broke the teams into two groups according to the difference between their passing and rushing abilities according to Advanced NFL Stats data. The typical playoff team over the past 14 years earned 62 more “expected points” during the season from passing than from rushing, and so teams that deviated significantly from that differential I defined as “one-dimensional.” And I found nothing, at least, based on this definition. The final margin of victory was not significantly affected by one-dimensional offenses. The 2008 Superbowl featured two such teams, the Patriots and the Giants. The Patriots were pass-heavy, garnering 200 additional expected points via their aerial assault on the regular season, while the Giants were a run-heavy team on the legs of Brandon Jacobs and Derrick Ward. Compounding the imbalance, the Giants had just the 15th-best defense in the league, and they won! This is but one example of how imbalance does not necessarily lead to an early playoff exit.

Defense wins championships

There might be something here. Obviously defenses matter, but the implication here is that they matter more than offenses come playoff time. While a good defense for the home team had essentially the same effect as a good offense, a good defense for the away team did make a significant difference. The top defenses in the league might be expected to shave two or three points off the spread compared to the additional points that top offenses in the league could create when playing on the road. This may be because home teams already have a strong built-in defense, sometimes referred to as the 12th man, while away teams must rely on their own skills and athleticism.

Rain and snow affect passing teams

Obviously blizzards and hurricanes do, but in general, this is not true. Not in the playoffs, anyway. The data suggests that rain and snow during the game may hurt the favored team in general, but not specifically a good passing team. The effect of precipitation on rushing and passing is essentially equal, and statistically insignificant. According to Wikipedia, the 2007 Superbowl between the Colts and dah Bears was the first to be played in the rain. Indianapolis had some guy named Peyton Manning, and the Colts featured the top passing offense in the league. Rain Schmain. The Colts won by 12 as Manning completed 66 percent of his passes for 247 yards, comparable to his season average of 269.3 yards. In the end, it seems as though a simple model may be the best. Teams that are good overall will be good in the playoffs, regardless of whether their value comes from rushing or passing, or from offense or defense. Teams playing on the road will always be at a disadvantage, but perhaps a little more so when playing under an unfamiliar ceiling.