Analysis of defenses often look at total yards allowed as a measure of how good a defense is. However, Tedy Bruschi thought that way is too simplistic.
So, with the help of ESPN, Bruschi has made his own defensive rankings that take several parameters into account:
Bruschi instead has put together his own formula, averaging what he views as the four key statistics: points allowed (only on defense, not including pick-6s or special teams scores), red zone defense (measured by percentage of touchdowns allowed vs. total trips), third-down percentage and total turnovers (created solely by the defense, not including special teams).
As one can see, this puts stats like total yardage into some much needed context. A defense which gives up 16 yards on a third and twenty was successful in a way that a defense which gave up 10 yards on a third and five was not. Red zone trips, and their frequency, are often the cause of a short field that the defense has to work with and turnovers are obviously a huge part of what creates swings in games.
Based on that, the top ranked defenses don't change that much from more traditional marks, except for the New England Patriots, who are the fifth best defense in the league according to Bruschi's rankings.
This is even taking into account the fact that the Pats are ranked 21st in 3rd down defense. The stat that keeps the Patriots high in Bruschi's ranking is turnovers, where the Patriots are tied for fifth with the Carolina Panthers and their stinginess within the red zone. The rest of the top five includes the Panthers, Saints, Seahawks and Chiefs.
Given the injuries to the defense, it is remarkable that the Pats have managed to have a good defense, let alone a great one. If the Pats can improve on third down conversions, then the rest of the league better look out.