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Defensive Efficiency through 8 games

The 2011 Crimson Tide defense has thus far lit up the stat sheets. As kleph has done a fabulous job showcasing in recent weeks, the Bama defense leads the nation in most defensive raw statistics categories, including both points/game (6.88) and yards/game (180.5).

However, as we've pointed out several times, raw statistics can be misleading in three key ways. First, they don't take into account the number of possessions or plays that occur in a given game, and there is a lot of fluctuation here that can pad the stats of an offense that typically has a lot of possessions, or a defense that doesn't have to face many possessions. Second, raw statistics don't take into account field position. An offense that typically gets great starting field position from its defense might be unfairly rewarded for easy scores, while a defense that consistently has to set up inside its own red zone might be unfairly punished. And finally, raw statistics often include "garbage time" possessions, plays and scores that have no real bearing on a team's actual strength.

While efficiency ratings help account for those factors and give a better picture of unit strength, fortunately (although perhaps not interestingly in this case) they tell a familiar story: Alabama's 2011 defense is dominant. The Tide has faced 78 non-garbage-time possessions this year, and has only conceded touchdowns on six of the them (7.7% of possessions) while forcing turnovers on ten of them (13.2%).

Those rates are stunningly impressive, perhaps even more so than the raw statistics numbers that are impressive in their own right. However, one note that should be made is that Bama hasn't really faced an elite offense this year, and aside from Arkansas every other opponent has had an outright weak offense from an efficiency standpoint. Looking at the F+/- efficiency ratings often cited by SBNation stats guru Bill Connelly, only two of the offenses the Tide defense has faced in 2011 rank higher than 69th out of 120 FBS teams: Arkansas at 23rd and Tennessee at 38th. And it should be noted that Tennessee is only that high due to the somewhat impressive performances of their offense with a healthy Tyler Bray under center; the post-Bray UT offense that Bama faced probably wouldn't rank in the top half of FBS.

With that out of the way, though, there's still really no knocking just how effective this defense has been on each possession this season. The table below indicates the "value added" by the defense on each possession against each opponent thus far this season. As those who have read our previous pieces on unit efficiency are aware, the value added is the amount of value added to the final score margin by an offensive, defensive or special teams unit on a given possession based on the expected value of that particular possession.

Opponent Opp. Off. Rank Possessions Value Added Per Possession
Kent State 114 15 + 1.93
Penn State 70 10 + 1.70
North Texas 120 8 + 1.49
Arkansas 23 10 + 0.93
Florida 69 10 + 1.47
Vanderbilt 96 7 + 1.34
Ole Miss 106 8 + 1.74
Tennessee 38* 10 + 2.10
OVERALL 78 + 1.62

Despite the surprisingly weak nature of the offenses the Tide has gone up against, you really can't argue with the effectiveness of the defense in each and every game. Remember, any positive number in terms of value added means the defense is producing net value for the team above what is expected based on the opposing offense's field position on each possession. When you see a number like +2.10 for the Tennessee game, that means the defense was in essence generating over 2 points in added scoring margin for the team each time Tennessee's offense stepped on the field. The +0.93 value against Arkansas was the smallest number, but considering Bama was going up against one of the top 25 offenses in college football, generating nearly a point in value every time you take the field is quite a feat.

So what can we take away from these numbers? First, they are tops in the country. Even when you adjust for the weakness of the offenses Bama has faced, the Tide still comes out as the clear #1 defense from an efficiency standpoint in the F+/- system, which adjusts all efficiency ratings based on the efficiencies of the opposing offenses. However, it must be stressed that Bama still hasn't faced an elite offense to really test the strength of this unit. That, of course, will change on November 5th when LSU comes calling with the 7th-most efficient offensive unit in the country.  What the efficiency numbers really do is validate what your eyes and the raw statistics have already told you. Even when you adjust for number of possessions, average field position, take out garbage-time possessions and even adjust for somewhat weak opponent strength, the Tide defense is performing more efficiently--so far--than any other defensive unit in the country.