CHAPEL HILL, N.C. – North Carolina’s 2015 defense may serve as the ideal poster child for the ever-changing statistical landscape that belongs to college football.
Offensive ingenuity in recent decades has prompted the rise of advanced metrics to accurately gauge the effectiveness and efficiency of programs utilizing entirely different schemes to win games. Statistics such as total offense and total defense have largely become meaningless due to the significant discrepancy in plays run per game across the country.
Look no further than UNC’s offensive stats over the past two seasons for a prime example. In 2014, the Tar Heels ranked 48th nationally in total offense (429.8) yet 66th in yards per play (5.56). In 2015, the Tar Heels ranked 18th nationally in total offense (486.9) yet led the country in yards per play (7.28).
UNC’s defense was saddled with similar discrepancies last season, ranking 96th in total defense (435.9) yet 56th in yards per play (5.50). The differential was not nearly as dramatic in the Tar Heels’ woeful run defense, ranking 122nd in rushing yards allowed per game (247.4) and T-109th in rushing yards per play (5.22).
One particular stat, however, shined amongst the rest: scoring defense. UNC ranked 42nd in scoring defense (24.5), demanding praise for the largest turnaround in the country by holding opponents to 14.5 points fewer than the year before.
In a world of various advanced metrics, scoring points, or not allowing your opponent to score points, is all that matters.
"We don't put a whole lot of stock in yards allowed anymore,” UNC head coach Larry Fedora told reporters at the ACC Kickoff media event in July. “Total yards or defense, that's not really even a stat we pay any attention to. The most important stat is scoring defense. That's the way we look at it.
“Now, there are other stats that play into that role: turnovers, all those different things. But scoring defense is No. 1. If you don't let them get in the end zone, it doesn't matter how many times you let them run up and down the field.”
The concept is simple. Let your opponents churn out as much yardage as they want between the goal lines, but don’t let them cross the goal line. That’s the underlying principle of Chizik’s “bend-but-don’t-break” defensive philosophy.
Theories are one thing; taking a theory and applying it to real life – in this case, a football field – is quite another. The statistics that might typically explain scoring defense prowess don’t shed much light on UNC’s success.
The Tar Heels ranked 67th national in Football Outsiders’ S&P+ defensive ratings, which is based around concepts such as efficiency, explosiveness and turnovers. FO also tracks defensive success rate, which tracks whether or not every play in a game is successful based on certain benchmarks (50 percent of necessary yardage on first down, etc.). UNC ranked 108th nationally by allowing opponents to gain necessary yardage on 46.2 percent of their snaps.
By more traditional statistical measures, the Tar Heels were solid in red zone defense percentage (79.3, 35th), but below average in red zone touchdown percentage (62.1, 79th) and red zone touchdowns allowed (36, 107th).
There are two relevant statistics available that pinpoint UNC’s defensive scoring success from a season ago.
Brian Fremeau’s data on field position over the past decade has indicated a strong correlation between starting field position and points scored. For example, a team starting a drive at its own 20-yard-line can expect to score 1.65 points, while a team at its opponent’s 20-yard-line can expect to score 4.59 points. In 2015, UNC’s opponents’ average starting field position was the 28.5-yard-line, good for 43rd nationally (mirroring UNC’s scoring defense rank of 42nd).
The other advanced metric is FO’s IsoPPP, which takes the defensive success rate mentioned above and determines the per-play value of successful plays. In other words, if a running back breaks off a 50-yard run, does he reach the end zone or is he stopped short? Despite a poor defensive success rate, the Tar Heels ranked fifth nationally in IsoPPP in 2015 after ranking 93rd in the same metric in 2014.
UNC’s opponents scored touchdowns on 24.8 percent of their drives (44-of-178), yet only scored eight touchdowns from outside of the red zone. Gene Chizik’s defensive scheme effectively limited explosive scoring plays and forced opponents to march down the field and earn their touchdowns the hard way.
“Why were we so good at that?” Fedora said. “Even if we gave up plays, and we didn't give up a whole lot of explosive plays, we pretty much held people. They had to work for what they got. For that reason I think we were probably good on scoring defense. I think we're going to continue to be that way.
"It's very important because that's the No. 1 stat for us defensively.”