The Green Bay Packers have received an influx of offense from an unlikely source this year: their defense. The defense has recorded seven touchdowns in only 10 games this season; no other team has more than four. They're also on pace to score the most touchdowns in a season since the 1998 Seahawks' defense, which scored 10.
The Packers' performance in scoring defense this season has been the best in recent memory. In 2007, the Packers scored four defensive touchdowns; in 2006, they scored five. They're on pace for 11 this year, more than the previous two seasons combined.
While Packers' fans and coaches will take the points any way they can get them, Green Bay's dominance over the rest of the league in this category raises a lot of interesting questions for the future. Will the Packers continue on this hot streak? What does it mean about their performance for next year? Are the Packers simply better than the league on returns, or is it just the combination of opportunity (the Packers lead the league in interceptions) and luck?
To answer all these questions, we looked at the interception, fumble and block returns for every NFL team since 1995. We also included safeties, which the Packers don't have any of yet this year, to get the total number of points each defense accrued.
From 1995 through 2006 (the latter chosen so that we can analyze the defense's performance versus the following season), the average NFL defense scored 15.98 points per season. The highest-scoring team was the 1998 Seahawks, who put up 60 points. Twenty-two teams, most recently the 2006 Saints and Chiefs, did not score a single defensive point.
The top 50 teams of the sample scored an average of exactly 35 points per season on defense, while the bottom 50 teams could muster only 2.4 points in their given year. If scoring defensive touchdowns were a sustainable, consistent skill, we'd expect to see the top 50 teams continue to put up big numbers, while the bottom 50 teams would continue to struggle to garner interceptions and fumble returns for points.
That's simply not the case. The year after their successful season, those top 50 teams averaged only 14.8 defensive points. The bottom 50 teams? They averaged 14.2 defensive points, essentially the same total. The correlation coefficient (which measures the relationship between two variables, with minus-1 indicating a perfectly dissimilar relationship and 1 a perfectly similar relationship) between the number of points a team scored in a given year and what the team did in the following season is minus-0.06; in other words, there's virtually no relationship between defensive points in a given year and the team's defensive points in the following season.
Should we even expect the Packers to stay on pace and end up hitting that 11-touchdown mark? History shows they're not likely to do so. No team since 1995 has had seven defensive touchdowns through 11 weeks. In fact, only two teams had six touchdowns: the 1997 Broncos and the 1999 49ers. In the final six weeks of the season, those two teams combined to score one defensive TD. Seven teams scored five defensive touchdowns in the first 11 weeks; they averaged two scores in the final six weeks. One of those teams was the aforementioned 1998 Seahawks, who had five touchdowns in their first 10 games and five in the final six. There is a very weak correlation of 0.09 between a team's defensive touchdowns in the first 10 games and in the final six. In other words, just because the Packers have shown a proclivity for taking returns to the house doesn't mean they're going to continue doing so.
The issue that should really worry Packers fans is what that means for next year's performance. Remember those top 50 teams from before? In the year that they scored the copious amounts of defensive touchdowns, they averaged 359.2 points scored. In the season after, they scored an average of only 340 points, a difference of 19.2 points. Of course, because they weren't stopping drives with interceptions that were returned for touchdowns or fumble recoveries, they also allowed more points, giving up 319.7 points in the touchdown-happy year, but that went up to 342.8 points the following season, a total of 23.1 more points allowed. In total, the difference between their points scored and allowed in the two seasons was 42.3 points.
What that could potentially mean for the Packers is encapsulated in what's known as the "Pythagorean Expectation," one of the many discoveries by baseball sabermetrician Bill James in the 1980s. He discovered that using a baseball team's runs scored and runs allowed when used in a variation of the Pythagorean theorem (you'll remember it from geometry class) was actually a better predictor of a team's wins and losses the year after than the team's actual wins and losses from the previous season. About 20 years later, statistician Daryl Morey discovered that you could do the same thing for football with the exponent changed from two to 2.37. (If you recognize Morey's name, you're probably a basketball fan: Morey is currently the general manager of the Houston Rockets.)
If we plug the points scored and allowed into this modified Pythagorean theorem, we'd expect the team laden with defensive touchdowns to win 9.1 games. In the year after, with new expectations for points scored and against? Those teams would win an average of only 7.9 games, a difference of more than a full win.
It'll be hard for the 2009 Packers to escape those trends. Of those top 50 teams, 32 scored fewer points in the ensuing season and 30 allowed more points than they had in the previous year. The Packers should not expect this trend to continue, either in the remainder of this season or into the future. They should view it as a bonus unlikely to be earned again in the future.
Bill Barnwell is an analyst for FootballOutsiders.com