Over at 538 http://fivethirtyeight.com/features/what-explains-the-carolina-panthers-rise/, Neil Paine has been using a method to evaluate teams.. He uses the expected pts from Football Reference, scales everything and rates pass off, run off, pass def, run def. He uses 100 as average, above that say 110-130 etc is good, 80-90 not so much.
I made a couple calculations and added a schedule adjustment. Cut to the chase and....
the 2014 Bills were 132 rating in pass defense, #1 in the NFL; we were 106 in run defense, again good
the 2015 Bills are 111 rating in pass defense, I was bit surprised; the run defense is 91, not so great.
These metrics show the defense has dropped approximately 15% in performance to last year in both defending the pass and run.