Let's start with a couple first-level measures. The Titans gave up 4244 passing yards in 2010, and only 3809 in 2011. That's not bad, especially when NFL teams as a whole gave up 123 yards more. Of course, the Titans also faced more passing attempts in 2010 than they did in 2011, 625 compared to 591. In Yards Per Attempt numbers, the Titans' defense improved from 6.79 YPA to 6.45 YPA.
Of course, not every called passing play results in a passing attempt, as sacks also play a role. As we know, the 2011 Titans weren't as good at getting to the passer as the 2010 version. The 2011 Titans had 28 sacks for a loss of 180 yards, whereas 2010 saw 36 sacks for 212 yards. With those sacks added in, the 2010 Titans gave up 4032 yards on 661 plays, whereas this year saw 3629 yards on 619 plays. In Net Yards Per Attempt numbers, the Titans' defense improved from 6.10 NYPA to 5.86 NYPA.
Then again, pure yards numbers ignore that passing attempts sometimes result in particularly valuable plays for either the offense or the defense, namely yards or interceptions. We'll instead get to my favorite measure, adjusted net yards per attempt. To make the adjusted net yards per attempt calculation, add 20 yards for every passing touchdown allowed and subtract 45 yards for every interception. The 2010 Titans allowed 23 passing touchdowns and had 17 interceptions. In 2011, the Titans allowed 21 passing touchdowns and only picked off 11 passing. In Adjusted Net Yards Per Attempt, the Titans' defense went from 5.64 ANYPA to ... 5.74 ANYPA. The pass defense actually ... got worse?
The answer, as always, is it's more complicated than that. Keep in mind that years aren't always the same. In fact, the overall NFL was more efficient at passing in YPA, NYPA, and ANYPA terms in 2011 than it was in 2010. Leaguewide, ANYPA was 5.73 in 2010 and 5.90 in 2011, so the Titans weren't the only team that got worse. For the New York Times' Fifth Down blog, Chase Stuart recently wrote an article where he compared each team's defense in ANYPA terms to the league average, and calculated the Titans allowing 5.74 ANYPA compared to the leaguewide average of 5.90 on 619 plays meant they allowed 98 fewer yards than an average team would. Running the same numbers for 2010, you find that edition of the Titans allowed 62 fewer yards than an average team would have. So, it now looks again like the secondary improved, but barely.
However, not all yards are created equal. A two-yard pass on third and goal from the two late in a game where the passing team is down by five is worth a lot more than a 20-yard pass on third-and-thirty when the passing team is down by 20 in the fourth quarter. Opposing passers are also not all created equal. It's not a big surprise when a quarterback like Drew Brees completes seventy percent of his attempts for 300 yards, whereas with a guy like Luke McCown you expect a much lower level of efficiency. In Football Outsiders' DVOA terms, the 2010 Titans pass defense had raw VOA of 12.5% and an opponent-adjusted DVOA of 6.4%, good for twelfth in the league. In 2011, the Titans pass defense had a raw VOA of 8.5%, and an opponent-adjusted DVOA of 13.9%, good for twentieth in the league. From this perspective, what looked like an improved passing defense instead looks more like a much kinder slate of opposing quarterbacks that made the pass defense look better when it instead was worse. Does this mean we should instead be throwing brickbats at Jerry Gray and pining for the return of Chuck Cecil?
Unsurprisingly, it's more complicated than that. As I've discussed before, the Titans' pass defense in 2010 was a tale of two half-seasons. In the first half-season, consisting of the first six games, the Titans got pressure on opposing quarterbacks (9.5% Adjusted Sack Rate), and the secondary played well. In the second half of the season, consisting of the final ten games, the Titans struggled to get to the passer (4.6% Adjusted Sack Rate, and the defense was shredded. I don't have the official DVOA numbers, but I'd estimate the Titans had a -26.2% pass defense DVOA the first six weeks and a pass defense DVOA of 26.1% the final ten weeks. Don't take those numbers as gospel, but they accurately reflect the direction and magnitude of the change in the Titans' pass defense. While the Titans' overall came out as average, or a little above it, they were either a great or abysmal pass defense, never average-ish like the final numbers made them appear.
My subjective impression would be the Titans' struggles against the pass were very likely to continue in 2011 unless they somehow found a way to rush the passer. They didn't find a way to get to opposing quarterbacks (5.0% Adjusted Sack Rate, good for 31st in the league), yet the pass defense was below-average but not close to abysmal like it was the second half of 2010. That certainly beat my expectations, and I think most other people's as well.
Overall, though, I think the Titans' pass defense rates as overall about the same as it was in 2010 (though that comparison doesn't make any sense because it doesn't make sense to think of 2010 as a unified whole), better than expected, but not good enough. A good pass rush may change that, but we'll first have to see how much the 2012 Titans' secondary ends up resembling the 2011 Titans' secondary, given that the top corner and top four safeties are all free agents.