UNC's Defensive Splits on 3rd Down

On December 11, UNC fired their defensive coordinator, and it was completely justified:

First, some quick hits on how bad our defense was overall (remember, there are 125 teams in Division 1 FBS):

• UNC ranked #117 in Total Defense, measured by YPG given up
• UNC ranked #119 in 1st Downs given up
• UNC ranked #122 (4th from last) in 3rd Down Defense, measured by percentage of 3rd downs converted by the opposing offense
• UNC ranked dead last in 4th Down Defense, measured by percentage of 4th downs converted by the opposing offense

To break it down further, when you remove the Liberty game and only include FBS teams, UNC gave up 1st downs on 50% of their opponents' 3rd downs. Only three other teams in all of FBS gave up 50%+.

Being at the ECU and Clemson games in person (two games in which we got torched defensively (although you could say that about most of our games)), I noticed that we often brought in a different personnel unit on 3rd down (we gave up 70.59% and 55.56% of 3rd down conversions in these games, respectively). To me, it seemed like we would do a decent job on 1st and 2nd down, and then give up the big play on 3rd down with the new unit. I get that different units have different specialties, and you've gotta get players rest, but if the same group continuously gets burned on 3rd down, why keep putting them out there?

I sought to see whether this hunch was backed by the numbers. I looked at every teams' breakdown of when they gave up their 1st downs. UNC gave up 30.06% of their opponents' 1st down conversions on 3rd down, meaning the other 69.94% came on 1st/2nd/4th combined. The FBS average for this figure is 28.80%; UNC ranked below average, at 80th in the country.

This is poor, but it's not as horrible as the other defensive rankings cited above. Another pattern I observed was how often we gave up a "big" play on 3rd and long. I defined 3rd and long as 3rd and 7 or more, and logged every defensive play on 3rd down (in the regular season). Of the 103 1st downs UNC gave up on 3rd down, 34.95% came on 3rd and long. When I include one less yard (3rd and 6), that proportion grows to 46.60%. A figure that close to half is atrocious. I also split 3rd downs of 10 or more, and opponents converted on 14.56% of these, meaning roughly 1 in every 7.

By every metric, UNC's defense was bad, and most notably bad on 3rd and 4th down. When you're one of the worst defenses in FBS across the board, the DC deserves to get the axe.

Sources of data:

Overall rankings: NCAA

Splits: CFBStats

Overall breakdown: TeamRankings

Play-by-play: ESPN