With five weeks in the books, every team in the NFL has made it through at least one-quarter of its schedule. Each defense in the league has faced north of 40 legitimate drives, and as a result, we can start taking the numbers that a given defense is producing with some respect.
Of course, four games' worth of numbers still can mislead you. Take last year's New York Giants defense, for example. Through four weeks of action, the Giants had the best pass defense in football. The reason? They played the Tampa Bay Buccaneers, Kansas City Chiefs, Dallas Cowboys and Washington Redskins. They allowed 330 passing yards to the one good team in the bunch but otherwise looked great. The Giants then kept it up with another solid performance against the Oakland Raiders, but in Week 7, they played the New Orleans Saints and got ripped to shreds by Drew Brees. Throw in an injury-riddled secondary and a front seven that quit on its defensive coordinator, and by the end of the season, the Giants had a below-average pass defense.
In this column, our goal is to combine both quantitative analysis and qualitative analysis to take what the numbers spit out and apply a dollop of context and common sense. So this week, let's look at some of the key shifts in defensive performance around the league and analyze how likely they are to stay changed. Keep in mind that all rankings are by DVOA; for full defensive rankings, click here.