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A statistical comparison: How much better is Notre Dame’s defense than last year’s?

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If Wednesday was spent comparing Notre Dame’s offense to those of the previous most-successful teams under Brian Kelly, then it only makes sense to spend Thursday looking at how the 2018 Irish defense rates against those from 2012, a historic unit; 2015, a team pushed by its offense; and 2017, presumably the most similar.

Of course, all those defenses had ebbs and flows, including last season’s November debacles. That is part of the game, and applies this year, as well. Though largely healthy, Notre Dame is currently worn out from a lack of depth at many positions (pretty much everywhere except defensive end) and the natural beating of a football season.

“I think our defense could get better in a lot of ways,” Irish junior cornerback Julian Love said Sunday. “The main component to that is rest and recovery. We’ve been grinding all season, and a lot of guys have been playing a lot of snaps.”

Love then specifically mentioned linebackers Te’von Coney and Drue Tranquill — the latter sitting right next to him and about to discuss his litany of injuries — as having played nearly every snap.

Even worn down, this defense shines in relation to its predecessors. Again, the five easily-understood yet quite-telling stats under consideration:

Yards allowed per play (YaPP): Pretty obvious as to why, right?
Yards allowed per pass attempt (YaPA): This essentially combines completion percentage and explosive plays into one metric, a quick measure of a team’s passing efficiency.
Rush attempts per game against (RPGa): In other words, an opposing offense’s confidence in its ground game and ability to dictate the game with it.
Third-down conversion rate allowed (3D % a): The ability to sustain a drive.
Turnovers forced (TF): Again, pretty obvious, right?

YaPP YaPA RPGa 3D % a TF
2012 4.78 5.98 27.85 36.52 % 23 (16 int.)
2015 5.57 6.85 36.23 35.11 % 14 (9 int.)
2017 5.05 6.27 37.08 35.27 % 20 (10 int.)
2018 4.53 5.35 33.67 37.82 % 20 (12 int.)

Given its state the previous season, last year’s defense was much-improved and well-regarded. Even in a vaccuum , it deserved that praise. And that makes this season’s showing that much more impressive. It obviously helped to return seven full-time starters, two part-timers (cornerback Troy Pride, defensive end Khalid Kareem) and a division-one starter elsewhere (safety Alohi Gilman). Only senior rover Asmar Bilal was truly  new to starting, and he has become a reliable commodity, not a question mark.

Returning that inventory does not diminish the accomplishment of dropping the average yards allowed per play by half a yard (10 percent, if wanting to look at it that way). The Irish have improved in every one of these categories save third-down conversion rate allowed, not the end of the world for a unit okay giving up long drives if they are forced to be methodical, mundane and, as often as not, misfiring in the red zone. Opponents have turned 33 red-zone trips into just 18 touchdowns, a 54.5 percent conversion rate and No. 35 in the country, similar to last season’s 52.1 percent (24-46) at No. 25.

Looking further back, Notre Dame’s defense in 2015 was a worthwhile one, but it hardly belongs in the same conversation as the rest of these. Its failure to force more turnovers led to the Irish having a negative-6 turnover differential (plus-8 in 2012; plus-3 in 2017; plus-5 in 2018). Even its raw scoring numbers show how much those 10-2 Irish needed their offense, giving up 24.1 points per game (12.77 in 2012; 21.5 in 2017; 17.25 in 2018).

And 2012, well, not much needs to be said about that defense. The rush attempts per game against figure stands out; Notre Dame opponents wanted no part of running into Lou Nix, Manti Te’o and Co.

Not that they very much want to now, either. Aside from the defense that led to national championship game appearance, 33.67 oppoisng rush attempts per game is the lowest of Kelly’s nine-year tenure. To give further context: Only once has it risen higher than 38.54, unsurprisingly the nadir of 2016 at 42.92.

Which only goes to show how far this defense has come, even from that oh-so-close 2015 season.