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, http://www.pro-football-reference.com/blog/wordpress/?p=156 and http://www.pro-football-reference.com/blog/wordpress/?p=157.

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)

HOME FIELD ADVANTAGE AND TIME ZONE CHANGES

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.

ALL GAMES

• 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.

Home Field Advantage and Passing

This is a follow up to the previous post, in which I looked at home and road won-loss records of all teams finishing between 6-10 and 10-6 for the years 1995-2006.

Here are the pass offense stats:

• below 6.0 - 18 teams; .576 at home; .299 on road
• 6.1-6.3 ypa - 34 teams; .563 at home; .419 on road
• 6.4-6.6 ypa - 43 teams; .583 at home; .407 on road
• 6.7 -6.9 ypa- 35 teams; .577 at home; .423 on road
• 7.0-7.2 ypa - 29 teams; .591 at home; .425 on road
• 7.3-7.5 ypa - 25 teams; .605 at home; .455 on road
• 7.6-7.8 ypa - 14 teams; .580 at home; .455 on road
• 7.9 and up - 12 teams; .667 at home; .469 on road

And here are the pass defense stats

• below 6.0 ypa - 22 teams; .591 at home; .438 on road
• 6.1-6.3 ypa - 35 teams; .636 at home; .443 on road
• 6.4-6.6 ypa - 46 teams; .598 at home; .428 on road
• 6.7 -6.9 ypa- 48 teams; .560 at home; .422 on road
• 7.0-7.2 ypa - 34 teams; .581 at home; .414 on road
• 7.3-7.5 ypa - 14 teams; .589 at home; .321 on road
• 7.6-7.8 ypa - 4 teams; .563 at home; .406 on road
• 7.9 and up - 7 teams; .464 at home; .339 on road

Teams improve their overall record as pass defense improves, but the difference between home and road performance stays fairly constant. The same is generally true of the pass offense as well. However, the 18 teams that were "poor" offensive passing teams (ypa 6.0 or lower) were horrible on the road while being decent at home.

As a result, I looked at teams who were below average in passing, while also being good at stopping the run. Here are the teams from the bottom passing offense groups of 6.6 ypa and below, sorted by their rushing defense numbers:

• 3.5 or below - 13 teams; .615 at home; .365 on road
• 3.6 - 3.8 ypc -30 teams; .594 at home; .350 on road
• 3.9 - 4.1 ypc -24 teams; .583 at home; .443 on road
• 4.2 - 4.4 ypc -18 teams; .528 at home; .396 on road
• 4.5 and up - 9 teams; .542 at home; .417 on road

To summarize, the top 2 groups on this list are the below average passing offense, but above average run defense teams. Those 43 teams had a +.246 difference in their home winning percentage versus the road winning percentage, which equates to almost 2 more wins at home on average over the course of a season.

It appears that these types of teams (bad offense, good rush defense) have a greater road disadvantage because of the offense, but a greater home advantage because of the rush defense, creating a greater home/road differential.

Home Field Advantage and Rushing

Using the pro-football-reference.com database, I looked at all teams that finished between 6-10 and 10-6 for the years 1995-2006. Why those records? It's partially arbitrary-- I could have just as easily narrowed it to 9-7/7-9, or expanded it to 11-5/5-11. But my goal was to look at the middle class of the NFL, teams that generally have some strengths but also some flaws. I felt this dividing line would accomplish that.

I then compared some of the standard rate stats, such as rush yards per attempt and pass yards per attempt, to see what team characteristics, if any, contributed to a greater difference between home and road winning percentages. This post will focus on the rushing stats, both offensive and defensive.

210 total teams finished between 6-10 and 10-6 during the 12 seasons reviewed, an average of over 17 per season--so slightly more than half the teams in the league on average. The entire group averaged winning .586 at home and .417 on the road, for a +.169 difference between home and road winning percentages. This would equate to +1.36 more home wins than road wins over the course of a 16 game schedule.

Here are the numbers divided by offensive rush yards per carry:

• 3.5 and below - 32 teams; .582 at home; .398 away
  3.6 to 3.8 ypc - 34 teams; .590 at home; .408 away
  3.9 to 4.1 ypc - 63 teams; .567 at home; .423 away
  4.2 to 4.4 ypc - 40 teams; .591 at home; .417 away
  4.5 and above - 41 teams; .611 at home; .431 away

Here are the numbers divided by defensive rush yards allowed per carry:

• 3.5 and below - 26 teams; .611 at home; .399 away
• 3.6 to 3.8 ypc - 47 teams; .625 at home; .380 away
• 3.9 to 4.1 ypc - 57 teams; .605 at home; .425 away
• 4.2 to 4.4 ypc - 41 teams; .537 at home; .419 away
• 4.5 and above - 39 teams; .548 at home; .460 away

There is basically no indication that simply running the ball better on offense, without any other info, either increases home field advantage or reduces road disadvantage.

Unlike the offensive numbers, the defensive numbers appear to show a difference between how the "middle class" performs at home vs. on the road, depending on whether the team is good at run defense or not. Teams that are not particularly good at stopping the run show less difference in how they perform at home vs. on the road.

If the strength of rush defense does increase home field advantage, there is a potential explanation. If a team is better at stopping the run, it is conceivable that such a team would be somewhat more likely to place its opponent into more 3rd and long situations. This might translate to a bigger advantage at home, where the offense is subject to crowd noise, than on the road, where the home crowd would presumably be quiet to aid the offense. Of course, we also need to keep in mind that this is looking at some of the variables. A team that is poor at stopping the run, yet finishes in the middle, likely has strengths in other areas to compensate. A team that is good at stopping the run, but does not finish in the upper tier, also likely has some other flaws that may be combining to create road disadvantage as well.

Next time, I will play with the passing numbers, overall offensive numbers and overall defensive numbers.