14 Nov 2012
Krishna Narsu: With Ras-I Dowling going on injured reserve and the Patriots' two starting safeties being injured, they've been very banged up in the secondary yet again. It seems to me to be an annual trend to see the team ravaged with injuries in the secondary. Is there any truth to this? What does Adjusted Games Lost tell us about how injured the Patriots have been in their secondary over the years? (I feel this is a problem that goes way back -- definitely as early as 2004 since that was the year they lost Ty Law and battled injuries in the secondary.)
In short, Krishna, there's plenty of truth to it. Here is a table showing the average yearly AGL for defensive backs from 2004 to 2011:
Remember that AGL measures the number of games lost to injury for meaningful players (i.e., not just starters) after adjusting for their status on the injury report. So, we can interpret New England's No. 31 defensive back AGL over the past eight seasons in two ways: (1) They've averaged the equivalent of losing one important defensive back for the entire season; and (2) They've suffered almost twice as many adjusted games lost as the average NFL secondary.
Of course, that's just an average. It's not like they've been decimated by injury every year -- or is it? The Patriots ranked better than 26th in defensive back AGL only twice over the past eight years. In a 2007 season that neared perfection in a myriad of ways, they ranked fourth with 1.23 AGL. Two years later, they ranked third with 2.96 AGL.
To take this a step further, I presume the underlying premise of Krishna's question to be that defensive back injuries have been a contributing factor to New England's ineffective pass defense of late. Well, I checked out the leaguewide relationship between defensive back AGL and pass defense DVOA over the eight year period, and found that the correlation equaled 0.08, which means that under one percent of the variation in the latter can be explained by the former.
That's not very surprising when we consider that (a) injuries to defensive backs don't necessarily transform a pass defense into football's version of a canned food drive, and (b) pass defense isn't just about what happens in coverage; there's also the pass rush.
In that vein, it turns out that New England's pass defense DVOA since 2004 can be summed up in the following way: As their Adjusted Sack Rate (ASR) goes, so goes their pass defense DVOA. (Feel free to check out Football Outsiders' Premium statistics database to see for yourself.) It's certainly not a one-to-one relationship, but it seems like as good a stats-based hypothesis as I can come up with on short notice.
In 2004 and 2006, high ASRs mitigated their high defensive back AGLs. In 2005, a low ASR combined with a high defensive back AGL -- the worst of both worlds -- produced the No. 31 pass defense DVOA. In 2007, a high ASR intensified the positive effect of health in their secondary.
Since then, however, middling ASRs year after year have produced no discernible pattern of pass defense performance. For instance, in 2009, a mediocre pass rush (No. 18 ASR) coupled with pristine secondary health (No. 3 AGL) resulted in an average pass defense (No. 16 DVOA). In 2010, though, a similarly mediocre pass rush (15th) resulted in a similarly mediocre pass defense (17th) despite nearly the exact opposite of pristine health in the secondary (27th).
The general interplay between AGL, ASR, and pass defense DVOA is definitely worth looking into more thoroughly when we have the down time to do so during the offseason; not to mention that we'll be able to see if New England's team-specific trend continued this year. As of this writing, they're in the middle of the ASR pack once again, and their pass defense DVOA is as well despite all the secondary injuries Krishna cited. In other words, it looks like 2010 all over again.
For now, though, there's an entire table of AGLs to chew on and discuss in the comments. Fire away.
20 comments, Last at 19 Nov 2012, 12:18am by Insancipitory