Domes and Homefield Advantage

Sports Illustrated's Tom Verducci is one of the best baseball writers around. In fact, he's probably my favorite. He respects the valuable research done by sabermetricians, but he also possesses the best characteristics of previous generations of baseball fans. His interest in and knowledge of the game are always evident, and he writes in an engaging style.

With the Super Bowl fast approaching, this week Verducci veered away from the diamond to cover North America's most popular sport. This in contrast to me, a football writer who this week wrote about tennis. Yeah, the biggest game in my favorite sport is a week away and I wrote about something else. Verducci, obviously, is a much smarter man than I am. Anyway, his article was about the huge home-field advantage enjoyed by football teams in domed stadiums.

In the piece, Verducci presented some statistics that, even if you know football pretty well, look very convincing:

* Four of the past six teams to reach the Super Bowl have been dome teams.

* The Colts and the Saints reached the Super Bowl with below-average defenses, ranked 18th and 25th, respectively.

* A dome team is 15 percent more likely to win at home than a non-dome team at home.

* In 2009, dome teams were .633 at home, while non-dome teams were just .551 at home.

* Peyton Manning has played much better in indoor playoff games than outdoor playoff games.

* Dome teams at home are 7-0 in the playoffs. Teams without a dome are 0-3 at home.

Taken together, that suggests pretty firm support for Verducci's case. For several reasons, though, you shouldn't take these numbers at face value. Let's address those points one at a time.

Four of the past six teams to reach the Super Bowl have been dome teams.

This actually isn't true. The Colts and Saints are both dome teams, and the Cardinals have a retractable roof, so we'll count them. The Steelers, Giants, and Patriots all play outdoors. That's three of the past six teams to reach the Super Bowl. In fact, only four of the last 16 teams to make the Super Bowl played their home games in a dome. The article cherry-picked a cutoff line that made the stats appear to support its thesis. This particular technique of distorting statistics is called multiple endpoints. In fact, I've just used it, too (four of the last 16). I stopped at 16 because one more year adds the 2001 Rams, weakening my point.

Let's go with a nice, round 20 teams: the last 10 NFL seasons. That brings us to five of the last 20 Super Bowl teams playing home games in a dome. This means 25% of Super Bowl teams were dome teams. That's almost exactly what we would expect: 22.6% of all NFL seasons this decade were played by teams with a domed stadium or a retractable roof. In the whole history of the game, only two dome teams (the '99 Rams and '06 Colts) have won the Super Bowl. Verducci is arguing trends, so make that two of the last 10 if you like. It's still 20%, which is about the expected rate, actually a little low. Next Sunday, it will become 27%: about the expected rate, just a little high.

The Colts and the Saints reached the Super Bowl with below-average defenses, ranked 18th and 25th, respectively.

I actually don't know what this is supposed to prove, but it's misleading. Teams with good offenses — especially good passing offenses — give up more yardage than teams with bad offenses, both because the pace of the game is faster and because the opponent passes more while it's trying to catch up. Pass plays gain more yardage than running plays do (averages are 6.2 yards and 4.2, respectively), so teams that are passed against more often yield more yards. The Colts and the Saints have the two best offenses in the NFL; we would expect them to give up a lot of yards.

The Saints forced 39 turnovers in the regular season, second-best in the NFL. They held opposing passers to a 68.6 passer rating, third-best. This was a gambling defense, willing to give up yards in order to make big plays. The New Orleans defense scored seven TDs this season. No other defense scored more than four (Eagles and Titans). New Orleans plays an aggressive defense that gives up yards, but creates opportunities. It ranked 25th in yards, but might have been one of the 10 best defenses in the NFL this season. Indianapolis ranked eighth in points allowed. These were both above-average defenses, not subpar as Verducci's article implies.

Peyton Manning has played much better in indoor playoff games than outdoor playoff games.

Manning indoors during the postseason: 6-3, 333 ypg, 20 TD, 8 INT.
Manning outdoors during the postseason: 3-5, 230 ypg, 7 TD, 10 INT.

Yes, obviously Manning has played better indoors during the postseason. THE COLTS PLAY THEIR HOME GAMES IN A DOME. All of those dome games were home games, and none of the outdoor games were. So there are two very good reasons for Manning to have better numbers in those indoor games:

1) Home-field advantage.

2) Playing an inferior opponent. In the playoffs, teams that were better during the regular season get home-field advantage. In those nine dome games, the Colts were playing teams that were, on average, 2.5 games worse than them in the regular season. When a team with 13 wins (12.9, actually) plays a team with 10 wins (10.4), at home, won't the team with 13 usually do pretty well? Wouldn't you expect the quarterback — especially if he is among the best QBs ever — to have good numbers? A .667 winning percentage, 333 yards, and 5:2 TD/INT ratio is not out of line for what you would expect from Manning in home games against competitive teams.

In contrast, all but one of those outdoor games (Super Bowl XLI, against one of the best defenses in recent memory) were at an opponent's home field. Playing against a team with home-field advantage, that six times out of eight had a better record, wouldn't you expect a player to struggle? Is .375, 230 yards, and a couple more INTs than TDs outrageous for Manning in road games against better teams? No. On the contrary, I think it's about what you would expect. Really, don't all quarterbacks do better at home and against weaker teams? Yes...

Tom Brady at home during the postseason: 9-1, 193 ypg, 15 TD, 8 INT.
Tom Brady on the road during the postseason: 3-2, 235 ypg, 6 TD, 6 INT.

Don't let the yards per game fool you: at home, Brady threw more TDs and fewer interceptions, and his team had a much better record. Are Manning's home/road splits a little larger than we might expect? Yes. Do they demonstrate any clear difference compared to other players in outdoor stadiums? No, they do not.

Verducci even compared Manning to Larry Walker, "a guy who hit .381 at Coors Field, but .282 everywhere else." He's basically saying that if the Colts had drafted Ryan Leaf and the Chargers taken Manning, Peyton would be Kerry Collins, the football equivalent of a .280 hitter. For years, Coors Field was the friendliest hitters' park in MLB and offered the biggest home-field advantage. That has never been the case for the Colts' home stadiums.

Coors, with its high altitude and huge outfield, improved statistics for all hitters. The unique park allowed the Rockies to establish an enormous home-field advantage on top of this. The Colts' domes (they moved into a new one in 2008) are not unique: they provide a normal home-field advantage. In the 192 regular-season games Manning has played for Indianapolis, the team is 70-26 at home and 61-35 away, an advantage of less than one game per season. Tom Brady, who plays home games outdoors, is 53-10 at home, 43-20 away. Let's get a chart going here, including the postseason:

		HOME			ROAD
		W-L		%		W-L		%		Diff.
Manning	76-29	.724		64-40	.615		18%
Brady	62-11	.849		49-23	.681		25%

So Brady and the Patriots, who play outside, have a larger home advantage than Manning and the Colts? Well, that would make Verducci's statistics totally disingenuous. It's like he was trying to use the mere fact of home-field advantage to convince readers that domes give an unfair advantage.

Yeah, pretty much.

A dome team is 15 percent more likely to win at home than a non-dome team at home. In 2009, dome teams were .633 at home, while non-dome teams were just .551 at home.

This is actually the same statistic, just expressed in different ways: .633 is 15% higher than .551. What we have to ask ourselves is whether 2009 just happened to be a good year for dome teams. The answer is yes. Dome teams had a winning record in road games this season, .528. Non-dome teams were .391 on the road. Verducci just as easily could have written this:

"A dome team is 15 percent more likely to win at home than a non-dome team at home, but 35 percent more likely to win on the road than a non-dome team on the road."

Dome teams were better than non-dome teams in 2009, period. The Colts, Saints, and Vikings weren't good because they play in domes. They're just great teams. Last year, the Titans, Giants, and Steelers weren't good because they played outside. They were just great teams. From these statistics, it appears that dome teams actually have less home-field advantage than outdoor teams. This is the most misleading statistic in the entire column, even worse, I think, than not being able to count to four. The exact opposite of what the column implies is true: dome teams had a smaller home-field advantage this season than outdoor teams.

Dome teams at home are 7-0 in the playoffs. Teams without a dome are 0-3 at home.

This is true. Even if you go back more than a year, dome teams tend to do better in home playoff games than outdoor teams do at home. Over the past decade, dome teams are 19-4 at home and 6-17 on the road in postseason play. Outdoor teams are 45-32 at home, 30-47 away.

			Home	Road
Dome		.826		.261
Outdoor		.517		.390

The differences are within the margin of error, but just barely. One of the simplest and most common ways of calculating margin of error, which yields about 95% certainty, is 1 divided by the square root of n. By that method, it is plausible that outdoor teams are better at home in the postseason than dome teams, and worse on the road. It is plausible, but it is not terribly likely.

Conclusion

Verducci used several statistics to support his argument that dome teams have an unfair advantage.

Four of the past six teams to reach the Super Bowl have been dome teams.

This isn't true. The author also used multiple endpoints to hide the fact that dome teams reach the Super Bowl at almost exactly the average rate.

The Colts and the Saints reached the Super Bowl with below-average defenses, ranked 18th and 25th, respectively.

Both teams gave up lots of yards, but neither truly had a below-average defense. I remain unclear on how this relates to his argument, anyway.

A dome team is 15 percent more likely to win at home than a non-dome team at home.

A dome team is also 35 percent more likely to win on the road than a non-dome team on the road.

In 2009, dome teams were .633 at home, while non-dome teams were just .551 at home.

This is the same stat as above. It's worth noting that those figures (15% and 35%) are only for the '09 season.

Peyton Manning has played much better in indoor playoff games than outdoor playoff games.

Of course he has, those were home games. Tom Brady's Patriots actually have a larger home-field advantage (outdoors) than Manning's Colts (in a dome).

Dome teams at home are 7-0 in the playoffs. Teams without a dome are 0-3 at home.

This is true. Over the past decade, dome teams do better at home, and worse on the road, than teams that play their home games outdoors.

The available evidence does not suggest that dome teams enjoy an unfair advantage compared to teams that play their home games outside. They have done a little better in the playoffs, but they don't reach or win Super Bowls at a higher rate than we would expect. In fact, they seem to have exactly the same home-field advantage as everyone else.

I hope it's clear that I continue to respect Verducci and enjoy his writing. He's writing about something outside his usual area of expertise, and I suspect the researchers helping him fed him some numbers they thought were helpful, not realizing how they undermined his argument. The numbers just don't support the assertion that domes offer any more home-field advantage than outdoor stadiums.

For years, domes were thought to offer less advantage than outdoor facilities. Cold-weather teams could win anywhere, the thinking went, while indoor and warm-weather teams fell apart in snow or wind. Older artificial turf models were associated with increased risk of injury. Now, with two dome teams making the Super Bowl in the same year, there are bound to be some people decreeing the opposite. Maybe it will become apparent over time that dome teams really do have some advantage over outdoor teams, but for now I think that notion is at least as baseless as the old idea that dome teams were at a disadvantage. The numbers just don't back it up.