Do team defenses tend to do better at home than on the road?

The quick answer is yes. After 13 weeks, the average team defense scores -420 SWP each game. But a defense at home averages only -387 SWP, while a road team averages -454, a differential of 67 SWP, or about the value of a touchdown. But that's not the real story here. You have to drill down a little deeper to find the interesting data.

Ten teams do more than 100 SWP better at home than on the road. Of these, six teams average more than 200 SWP better at home. And the Vikings are 367 SWP stronger in the Metrodome than they are on the road. In fact, of their first twelve games, their worst home week produced -355, while their best away week was over -390. What could cause such a disparity? Is it real, or just a statistical fluke? Consider these thoughts:

• Could it be strength of schedule? Minnesota's non-divisional home opponents have included Washington, New Orleans, and Cincy - not particularly powerhouse offenses. Road foes have included St. Louis and Dallas. But three division foes have played Minnesota both home and away (Detroit, Tampa Bay, and Green Bay), and the Vikes have averaged 244 points better against these three teams at home vs. on the road. So while schedule may have a small role to play, it certainly doesn't look anywhere close to being a dominant factor.
• Could it be a domed stadium advantage? One of the Vikes' road games has been in a dome - at Detroit - and that was a -396 SWP game, which is one of the better road outcomes. I suppose it could be turf related, too, although Dallas has artificial turf, and they burned the Vikings for -873 SWP.
• The Dallas game fell on a short week for practice, so maybe it is an anomaly for that reason. Excluding that game from the average improves the road average by almost 60 SWP, but that still leaves a substantial disparity.
• In any short series of numbers, there can always be extraneous factors that have a material impact. For example, when Minnesota played Detroit in Minneapolis, they faced Charlie Batch in his first pro start, whereas Batch was probably more comfortable 5 weeks later when the Vikes came to Pontiac. Also, they played in St. Louis on week 2, when the Rams were much healthier (e.g., Isaac Bruce produced 480 SWP). Even so, 367 SWP per week is an awful lot to explain away.

Who are the other five teams who are more than 200 SWP better at home than on the road? Here's the chart of the top teams:

Defense SWP Averages - Home vs. Away
Team	at Home	Away	Diff.
Minnesota	-217	-584	367
Miami	-143	-432	289
Kansas City	-280	-532	252
Arizona	-308	-531	223
Indianapolis	-468	-671	203
NY Jets	-242	-442	200

Are there distinctive factors that might explain these teams? Indy is another dome team. Miami and Arizona may prefer warm weather. The Jets play on artificial turf at home. Kansas City - noise, perhaps? The Chiefs also had that one night game against Seattle played in a monsoon. (But even if Seattle had put up average offensive numbers that night, KC would still have a SWP differential greater than 200.)

How about the other end of the spectrum? Are there teams that do worse at home?

Yep. In fact, 10 teams (one-third of the league) do worse at home than on the road. The most proficient road warriors are the Ravens, who average -502 in Baltimore, but -366 on the road, for a road improvement of 136 SWP. The 49ers are almost on par with that, averaging -456 at home, and only -333 on the road, for a 123 SWP differential. I'm not sure what to make of these. The next four teams - Washington, San Diego, Philadelphia, and Chicago - are all only about a touchdown better on the road than at home.

So what to make of all this? As I've said before, beware of reading too much into statistical analysis when the number of games is so low. If you repeatedly flip a coin 12 times, you should expect to get 10 or more heads almost 2% of the time. But Minnesota's results sure look like more than a statistical anomaly. I'd say this is certainly a factor that warrants consideration when you're picking your defense for the week. Just not the only factor.

Here's the entire table through 12 weeks, sorted by differential:

                     Average SWP per Game
Team                Home     Away     Diff
Minnesota           -217     -584      367
Miami               -143     -432      289
Kansas City         -280     -532      252
Arizona             -308     -531      223
Indianapolis        -468     -671      203
New York J          -242     -442      201
Detroit             -353     -531      179
New Orleans         -355     -502      147
Pittsburgh          -298     -432      134
Cincinnati          -516     -619      104
Oakland             -302     -381       79
Jacksonville        -392     -468       76
St. Louis           -409     -476       67
New York G          -414     -476       62
Buffalo             -372     -421       49
Carolina            -490     -537       47
Atlanta             -332     -376       44
Tampa Bay           -403     -438       35
Tennessee           -444     -463       19
Green Bay           -377     -383        6
Seattle             -380     -368      -12
Denver              -363     -343      -20
Dallas              -406     -382      -24
New England         -435     -408      -27
Chicago             -479     -424      -55
Philadelphia        -506     -450      -55
San Diego           -400     -327      -73
Washington          -586     -504      -82
San Francisco       -456     -333     -123
Baltimore           -502     -366     -136

30 team average     -387     -454       66