How good is Baltimore's defense?

Ray Lewis is still the heart behind a Baltimore defense that appears as dominant as ever. Patrick Smith/Getty Images

Stop me if you've heard this one before. The Baltimore Ravens are 4-1, atop their division thanks primarily to a defense that's allowing a league-low 4.5 yards per play. Since Ray Lewis' fourth season, when they moved Rod Woodson to safety and drafted Chris McAlister to replace him at cornerback, the Ravens' defense has never finished below sixth, according to our DVOA rankings. That's 12 straight years of defensive dominance, with the trend continuing into 2011.

Currently, Football Outsiders has the Ravens' defense with a trifecta ranking of No. 1 overall, No. 1 against the pass and No. 1 against the run. No defense has ever finished the season at that level of play-by-play efficiency across the board, so what we're witnessing is possibly the best defensive performance since 1992, and an even better Ravens performance than their legendary Super Bowl-winning defense of 2000. The question, therefore, becomes, "Can they keep it up?"

From a statistical perspective, the No. 1 DVOA defense after six weeks has generally come back to the pack for the remainder of the season. On average, it has been 30.8 percent better than the average defense through six weeks but 12.5 percent less efficient the rest of the way. In terms of rankings, this has meant an average drop from first to third. In other words, if history is any indication, a defense at the top of the league after five or six games hasn't typically remained there after 16 games.