Does Cold Weather Change the Game?

Recently I looked at the importance of various phases of the game in winning playoff games. I analyzed and compared regular season games featuring only playoff-caliber opponents and playoff games themselves. This analysis began with the question of whether defense really does win championships, but since has focused on broader comparisons as well.

In the last post, I found that over the past five seasons teams with the better run defense won slightly less than half of games between playoff-caliber opponents. But in the playoffs, the team with the better run defense won 67% of the time. With 114 regular season match-ups between playoff-caliber teams and 55 playoff games during the period studied, the difference may only be due to chance. However, in the comparison of regular season games and playoff games, pass offense, pass defense, and run defense did not show any differences nearly as large as run defense.

[Edit: Based on a 2-sample unpaired t-test, the difference in winning percentage of the better defense (67% vs 48%) is indeed statistically significant at the p=0.02 level. In other words, the sample sizes are large enough to say it is extremely unlikely the difference is by chance.]

Here is the table from the previous post. The winning percentage of the team with the superior stat is indicated. For example, the team with the better offensive passing efficiency won 52.6% of the regular season "good vs. good" match-ups and won 63.6% of playoff games.

StatGood vs Good Playoffs
O Run45.645.5
D Run48.267.3
O Pass52.663.6
D Pass51.856.3
O Int Rate50.958.1
D Int Rate55.358.1
O Fum Rate55.340.0
D FFum Rate54.454.5
Pen Rate47.352.7

One possible explanation for the difference in the importance of run defense could be the weather. The playoffs are played in January when the weather is cold and often windy in most NFL cities. Teams might bias their play selection toward the run because of the perceived increase in passing difficulty.

To test if weather is the reason for the observed difference in the importance of run defense in the playoffs, I analyzed regular season games played between any-caliber opponents in cold weather. Without direct temperature and wind data for each game, I defined 'cold weather' as being played outdoors in December in a city that averages below 40 degrees wind chill. There were 118 such games between 2002 and 2006.

(I also looked at just the games between playoff-caliber teams played in the cold. However, there were only 12 such games, so the results are not very meaningful. I also expanded the definition of playoff-caliber to 9+ win teams, for which there were 26 games. I'll list both results anyway in case anyone is curious.)

Below is the table of results. Again, the percentage of games won by the team with the superior stat in each category is listed. The first column (Reg. Season) is for all regular season games and all opponent types (n=1280). The second column (In Cold) is for all games played in cold climates (n=118). The third column (9+ Wins) is for games played in cold climates between teams that ultimately finished with 9 or more wins (n=26). The last column (10+ Wins) is for games played in cold climates between teams that finished with at least 10 wins (n=12).

StatReg. SeasonIn Cold9+ Wins10+ Wins
O Run55.055.445.850.0
D Run50.048.858.350.0
O Pass63.865.666.766.7
D Pass59.860.445.858.3
O Int Rate59.561.054.233.3
D Int Rate59.459.433.333.3
O Fum Rate60.865.054.233.3
D FFum Rate58.061.354.250.0
Pen Rate54.

I'll address the results from the first and second columns. The importance of each stat appears about the same in cold weather as in all games. Other than home field advantage, fumble rates are the only stats that indicate any significant difference in cold weather.

Focusing on run defense, we see that the team with the superior run stopping ability only won 48.8% of the 118 games played in cold weather. This rate is very close to the overall rate of 50.0% for all regular season games. This suggests that it is not the weather but some other factor in the playoffs that may enhance the importance of run defense.

The increased importance of home field advantage in cold weather is probably due to the 'dome at cold' effect, in which dome teams tend to have very little success playing outdoors in cold weather.

Although nothing was conclusively proven with this analysis, there are indications that playoff football is different than regular season football. Although the sample sizes weren't large enough to make solid conclusions regarding most variables, there is enough evidence to suggest that the playoffs comprise a special set of circumstances that may change the dynamics of the game. The level of competition, the weather, the prospect of elimination, or other factors may influence strategies and performances.

At the very least, we can see how fans may perceive defense as being more important in the playoffs. Whether there is truly a systematic link between run defense and playoff success, or it is only the randomness of a small sample, may not be relevant. We have witnessed teams with stronger run defenses win more playoff games. It is apparently, if not in reality, the most important part of the game come January.

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