49ers Quarterly Statistical Report: II. Defense

AUTHOR'S NOTE: Sorry for the delay in getting Part 2 up on the site. I had some random computer issues yesterday that decided to not resolve themselves for an inordinately long time. This post is really short because there's no telling when it'll happen again.

Welcome back for Part 2 of my review of the 49ers' advanced statistics through the first quarter of the 2010 season. In Part 1, I detailed the Niners' overall and (truly) offensive DVOAs, and concluded that (a) the 49ers should probably be 1-3 right now on the basis of their statistical profile; (b) the pass offense through 4 games has declined considerably in comparison to their 2009 stats; and (c) the addition of Mike Iupati and Anthony Davis has yet to register an impact on the 49ers' run offense stats.

I also mentioned in passing that the defense's inability to perform up to expectations thus far this season has been an overlooked drag on the team overall.  Today, I'll delve a little deeper into that topic.

DEFENSIVE RANKINGS - OVERALL

So, without further ado, here are the defense's DVOA stats and rankings look overall and by type of play, as well as how they've changed from last season (bold = top 8 in the NFL; italics = bottom 8):

Just to remind you about how to interpret defensive DVOAs, negative percentages are good because they mean that the defense has made opposing offenses have a lower play success rate than the league average. For instance, the 49ers' 3.4% Defense DVOA means that the unit has allowed their opposing offenses' plays to be successful 3.4% more often than the NFL average.

After the jump, I'll discuss this table, and present some defensive situational splits through the first 4 games...

As you can see, the pass defense is entirely to blame for the unit's overall decline from their elite status in 2009. Whereas the run defense has continued its efficient play this season, the pass defense is allowing the opposing offenses' plays to be successful over 30% more of the time than they allowed last season. Without getting on my soapbox too much here, when we couple this nosedive in pass defense efficiency with the decline in pass offense efficiency that I detailed on Wednesday, the 49ers' 0-4 start represents the poster child for "it's the passing game, stupid!" Back in my team win prediction post, I mentioned how the change in team wins from one season to the next tracks very closely with changes in Pass Offense DVOA and Pass Defense DVOA. Thus far in 2010, the Niners' running game on both sides of the ball has performed at nearly the exact same level of efficiency as it did in 2009, but the team has a much worse winning percentage. Why? Because it's the passing game, stupid!

Before getting into the situational splits, which explain some of the details of the pass defense's problems, there are few advanced stats that give us some general hints. First, the Niners' Defensive Adjusted Sack Rate (ASR), which measures the frequency with which a defense sacks opposing QBs, has dropped from a 3rd-ranked 7.9% in 2009 to a 16th-ranked 5.9% through 4 games in 2010. Getting to the QB less frequently makes the jobs of the Niners' pass defenders that much more difficult.

Speaking of the pass defenders, the second advanced stat that helps explain the Niners' pass defense problems is Pass Defense DVOA vs. various types of opposing receivers.  They weren't that good last year vs. #1 WRs (5.9%, 17th) or vs. RBs (15.5%, 23rd), so the fact that they're really bad this year simply speaks to the continued unimportance of Nate Clements (just kidding), and the perpetual enigma that is "how to defend a screen pass." What's more interesting to me, however, is what's changed. Namely, the 49ers have fallen off a cliff in terms of defending WRs besides the #1. Their Pass Defense DVOA vs. #2 WRs has dropped from a league-best -33.2% in 2009 to a 23rd-ranked 18.6% in 2010. Even worse, their DVOA vs. all other WRs ranks dead last in the NFL at 64.0% after ranking 6th last season (-18.9%). What explains this? Did Shawntae Spencer and Tarell Brown forget how to play? Are the Niners so focused on stopping the #1 WR - which they apparently don't do well either - that other teams are just taking what the defense is giving them? I look forward to your comments on this one.

DEFENSIVE RANKINGS - DOWN SPLITS

To investigate this pass defense mystery even further, here are the defense's pass and run DVOAs by down (bold = top 8 in the NFL; italics = bottom 8):

And here are the defense's DVOA down splits by distance (Short = 0-3 yards; Mid = 4-6 yards; Long = 7+ yards):

Pretty much anyway you slice it, the 49ers' defense has gotten worse across the board. And this isn't a situation like the one I discussed vis-à-vis the offense wherein 3rd down success masks an otherwise crappy unit in one season, and then the crappy unit gets unmasked the next season because they can't duplicate the 3rd down success. In 2009, the Niners' defense was awesome no matter the down, distance, or type of play. This year, they're the opposite of awesome, especially the pass defense.

What's concerning to me about the pass defense is that many of these stats suggest they're bad when they don't know the other team is going to pass. For instance, their worst down is 2nd down, and their worst down and distance is 3rd & Mid, both of which are game situations where the opposing offense isn't limited in their play-calling. Similarly, the defense is incredibly good on 3rd & Long - when the opponent is sure to pass. Is this a simple case of making the wrong defensive calls at the wrong time? Is it that opposing offensive coordinators have figured out the defense's play-calling tendencies? Is this related to the decline in pass rush I brought up earlier, such that they're finding themselves in a lot of down-and-distance situations where they can't bring the heat? What do you think?

BOTTOM LINE

No bottom line necessary here what with the brevity of the post. I'll have a lot more for you after Week 8, when the sample sizes are much larger. Oh, and if you're wondering about the special teams, yeah...sample size is ridiculously small there as well. But, hey, if you want to ask about any special teams stats in particular, fire away in the comments.

*DVOA statistics used to produce this article were obtained from Football Outsiders.