Of all the numbers generated and stats compiled in 2015, the one that resonates in this particular corner of the press box is this one:

Points Allowed Per Game - 19.9.

That was good enough to rank the 2015 Steelers 11th in the NFL in that department.

That was just a little bit more than a field goal better (3.1 points) than the Steelers had managed in 2014 (23.0 points allowed per game), when they'd finished a disappointing 18th among the NFL's 32 teams in that category.

But it was also the type of improvement not many if any were likely anticipating at the outset of the season, other than, perhaps, the defensive players and the defensive coaches.

That 19.9 figure is attributable to and reflective of many things, all of which bode well for the Steelers heading into 2016.

Among those:

-Defensive end Cam Heyward's relentless effort and presence on and off the field, which helped establish a fly-to-the-football and a whatever-it-takes mentality.

-Defensive end Stephon Tuitt's continued development on the way to becoming a dominating player up front.

-Linebacker Ryan Shazier scratching the surface of what he's consistently capable of now that his understanding of assignments and how offenses intend to attack the Steelers in a variety of situations is catching up with his athleticism.

-How big of a difference a timely sack or a timely turnover was able to make for a defense that still gave up its share of yards but more often than not found ways to work around that, especially in the red zone.

-The versatility and creativity of a defensive staff that wound up without cornerbacks Cortez Allen and Senquez Golson and safety Robert Golden to rely upon for a variety of reasons and still found ways to, for the most part, keep it together on the back end.

-An expectation that the run would be stuffed, especially close to the goal line and even if the other team was running an outside-zone scheme.

-An all-for-one mentality and chemistry that allowed for the sharing of one of the starting cornerback positions and both outside linebacker spots without any public complaining about roles or playing time.

-A healthier Mike Mitchell doing at free safety in 2015 what the Steelers had hoped he would more often when they brought him in as an unrestricted free agent in 2014.

None of that was a given on Sept. 10 at New England.

But throughout that night and the days and nights that followed the defense kept finding ways to get better.

And it did so while turning significant pages in terms of personnel and philosophy while transitioning to first-year coordinator Keith Butler and away from iconic defenders such as Ike Taylor, Brett Keisel and Troy Polamalu.

None of what was achieved on defense this season should be underrated or underappreciated.

And for a change there's reason to suspect this offseason that the 19.9 figure will be lower rather than higher next season.