Monday, April 23, 2007

Temperature Differences and Home Field Advantage

Back in October 2006, the pro-football-reference blog posted a two part guest post I prepared on the effects of travel distance and climate here, and

This is a refinement of that information, plus the info with all games, not just divisional.

I tweaked the methodology on climate slightly. Previously, I used the average temperature for the months of September through December for each outdoor city, and the difference between the averages for two cities. The only slight problem with this is that not all cities with similar average temperatures have similar climates. To try to capture this better, in this post, I used the average monthly difference between the cities in question, then re-sorted the data.

An example of this is Seattle and Chicago. These cities are within 1 degree in average temperature over the whole football season. However, Seattle is the coolest city in September, while Chicago is much warmer, but by the end of the season, the numbers have reversed. My new methodology was to look at the absolute difference in degrees for each month, and average them. The other refinement was a slight tweak to the dome average temperature, I used 72 degrees (the data supported this, as like the outdoor vs outdoor games, the home field advantage between dome and outdoor was weakest against teams between 65 and 76 degrees).

I have included dome teams, but for now, I am excluding Denver. Denver is a case study on its own. Here is the data for all other divisional games, using the new climate methodology.

within 5 degrees difference, monthly average: 289-259-1 (0.527)
5.1 - 10.0 degrees difference, monthly average: 303-267-1 (0.532)
10.1-15.0 degrees difference, monthly average: 136-98-0 (0.581)
15.1-20.0 degrees difference, monthly average: 199-126-0 (0.612)
20.1-25.0 degrees difference, monthly average: 144-85-0 (0.629)
25.1 + degrees difference, monthly average: 72-47-0 (0.605)

Again, the effect of climate differences in divisional games is real and fairly strong. However, it is at the similar temperatures, not the extreme ones, where the difference is noticeable. Once divisional opponents are outside a 10 degree difference, the home field advantage increases dramatically, and beyond 15 degrees, it is fairly constant.

However, the effect is not the same with non-divisional games. Here is the overall data for all games played between 1986-2005 (except involving Denver) using the same criteria.

within 5 degrees difference, monthly average: 575-453-4 (0.559)
5.1 - 10.0 degrees difference, monthly average: 653-498-1 (0.567)
10.1-15.0 degrees difference, monthly average: 450-318-0 (0.586)
15.1-20.0 degrees difference, monthly average: 529-364-1 (0.592)
20.1-25.0 degrees difference, monthly average: 227-158-0 (0.590)
25.1 + degrees difference, monthly average: 119-77-0 (0.607)

So, there is a split between divisional games and non-divisional games. The overall effect still appears when we combine both. Why is this? Other than a random split? Two guesses:

First, the NFL may schedule divisional contests at a higher frequency both early and more importantly, later in the season (they do have to get 2 in vs each divisional opponent), and these are the same times when weather differences will be more extreme.

Second, maybe there is also a familiarity component to reducing home field advantage, so that visitors from similiar climates, who are also familiar with the specific venue, are better off than visitors from similiar climates, but unfamiliar with the specific city/stadium.

I did look at all cases since the merger where a team stayed in the same city it had been in for the previous 5 years, but moved into a new (but same climate) venue, to see what effect it had on home field against divisional opponents from a similiar climate. This would include: Buffalo (1973), NY Giants (1976), NY Jets (1984), Washington (1997), Cincinnati (2000), Pittsburgh (2001), New England (2002), Detroit (2002) and Philadelphia (2003). The sample size is not large enough to say anything with statistical certainty, but it was interesting that the home field advantage against similiar divisional opponents spiked in year 2 following the move, and disappeared thereafter.

These teams went 12-9-1 at home against their "climate" rivals in year 1 after the move, and an incredible 18-4 at home in year 2. They won about the same number at home and on road in the years leading up to the move, and reverted to this pattern in years 3-6 following the move. So, there may be something to the familiarity thing. I may also try to look at conference rivals who played at the same venue in more than 2 consecutive seasons, division rivals after relocation, and new divisional rivals (such as with realignment in 2002).

Finally, here are the point spread records since 2002, of the home team in divisional games, sorted by temperature difference.

Within 5 degrees on average: 59-75-2 (.441)
5.1 to 10 degrees on average: 56-76-4 (.429)
10.1 to 15 degrees on average: 48-48-4 (.500)
15.1 to 20 degrees on average: 25-15-0 (.625)
20.1 degrees or more on average: 30-30-0 (.500)

Friday, April 13, 2007


Some random NFL kicker items from the 2006 season:

First, did the NFL juice up the ball last year for kickers? I looked at the touchback stats for all kickers who kicked off at least 50 times in both 2005 and 2006. Here are the results:

2005: 179 touchbacks out of 1,847 kickoffs (9.7%)
2006: 250 touchbacks out of 1,824 kickoffs (13.7%)

Of the 25 kickers qualifying under my criteria, 13 of them showed significant improvement in the touchback rate, 10 were roughly the same, and 2 declined (Rackers and Janikowski). This also does not take into account other veteran kickers who kicked off last year after not kicking off the year before, who also showed a significant increase in touchbacks over their most recent kickoff performances of prior seasons.

In line with that, kickers are aging like fine wine. If they were baseball players, they would probably be getting accused of performance enhancement. I don't know what all the reasons are for kickers in the NFL, and I am sure there are multiple factors. Here are the numbers for the 11 kickers who were age 33 or older at the start of last season (Andersen, Carney, Stover, Elam, Kasay, Vanderjagt, Wilkins, Nedney, Mare, and Vinatieri).

40-49 yard field goals
2006: 76 of 96 (79.2%)
rest of career: 874 of 1270 (68.8%)

50+ yard field goals
2006: 16 of 30 (53.3%)
rest of career: 226 of 430 (52.6%)

Kickoffs (excluding Elam and Andersen)
2006: 82 of 601 (13.6%)
rest of career: 919 of 6689 (13.7%)

Monday, April 2, 2007


How much does the change in time zone effect the home field advantage?

To attempt to come up with an approximate answer to this question, I used the home team wins and losses in all games since 1986, sorted by climate and distance. I eliminated games involving dome teams (for now) because I wanted to isolate the effect without the noise coming from the advantage of outdoor teams vs. dome teams. I also (for now) eliminated games involving Denver.

Denver clearly has a strong home field advantage. Denver plays all of its games against opponents from different time zones, and some of the advantage is likely due to this. But Denver also plays opponents from different elevations and weather patterns as well. So, for now, we will look at how time zone changes affect other teams, to get a sense of how much of Denver's advantage is due to this effect.

Here is the data for games between outdoor teams from the same time zone versus one time zone difference, versus two or more time zone differences.

  • Same Time Zone -- 759-603-2 (.557)
  • 1 Time Zone Diff -- 329-219-3 (.600)
  • 2+ Time Zone Diff-- 451-317-1 (.587)

The home field advantage is weaker when two outdoor teams are from the same time zone. However, while the advantage increases with a change in time zones, it does not appear to continue to increase with additional time zone changes. When we weight the time zone changes by cross-referencing against climate changes, the effect of playing an opponent from a different time zone versus one from the same time zone is roughly an increase in expected winning percentage of +0.035, but the effect of additional changes in time zone is negligible.

In games involving Denver and another outdoor team, the home team is 167-87 (.657) since 1986. In all other outdoor vs outdoor games since 1986, the home team is 1539-1139-6 (.575). Thus, Denver has had an advantage of +.082 above a normal outdoor vs. outdoor situation. Roughly +0.047 of this, then, is due to either random chance due to the smaller sample size, or to legitimate factors other than time zone changes, such as elevation changes and environmental changes between Denver and the opponent.

This research also supports that the effect of cross-country travel is overstated. Changing from the Eastern to the Pacific Time Zone has been no less a disadvantage than changing from the Eastern to Central Time Zones.

Conversely, the effect that coming from a similar environment and the same time zone has on reducing home field advantage is understated. In a future post, I will look at this further by looking at "against the spread" data.