There's been a lot of offseason chatter between fans and the players themselves over the true DBU. We try and get to the bottom of it, but if you want to skip ahead... it's LSU.
The method for the secondary is two parts: I looked at a team's pass defense stats and then compared it to a team's pass deflections per attempt and completion to try to assign responsibility or blame for the pass defense to the secondary. I also looked at interceptions/PD to look at the equivalent of "fumble luck", as defenses will tend to intercept the ball on 21% of its pass deflections. Anything more or less varies wildly from year to year and can be attributed to luck, not skill.
1 LSU. Let's take a second to appreciate just how terrific LSU's pass defense was last season. It ranked 1st in the SEC in completion percentage, passing yards, yards/attempt, and passer rating. All this without the benefit of a pass rush. The Tigers had a PD/Completion rate of 34.17%, off of the top spot by four-hundredths of a percent. The only stat in which the secondary did not dominate was interceptions, mainly due to a 14.71% INT/PD rate. With just normal luck, LSU intercepts five more balls. Oh, and the team returns its top corner, its top safety, and a senior leader. Far and away the best unit in the conference.
2 Florida. This is not to say that Vernon Hargreaves is not the best corner in the conference... he is. He's a machine back there, and Florida is right up there with LSU in PD/Comp and PD/Att. The difference here is that while LSU allows 5.5 YPA, Florida is at 6.0, a pretty large gap. It's still an elite unit, just one step behind.
3 Georgia. You know who ranked second in most pass defense categories (total yards, rating, YPA)? Not Florida, but Georgia. UGa's PD/Comp is a more modest 27.45%, just above league average, and the secondary did benefit from a pretty stout linebacking corps. Still, they return three starters from one of the best pass defenses in the SEC.
4 Tennessee. The Vols return every starter from one of the top secondaries in the SEC. They were prone to letting up the big play, but they kept opposing passers to 53.3% completion rate and they were hovering near 30% in PD/Comp. Also, they boast a corner in Cameron Sutton who was every bit as screwed over in the preseason All-SEC voting as Tre White.
5 Alabama. Few teams in the SEC gave up more passing yards, but that's mainly a function of protecting leads and teams trying to pass to catch up. Bama's 6.4 YPA ranked 6th in the conference, closer to their actual performance level. They also were plagued by a poor INT/PD ratio, which should improve this season. OK, Cyrus Jones shouldn't be 1st team All-SEC, but this is a solid unit.
6 Ole Miss. And here comes the crash. Last year, the Rebels picked off 22 passes at a rate of 35.48% INT/PD. The SEC average was just under 21%, making the gap between Ole Miss and league average roughly the same as LSU's entire INT/PD rate. This stat will regress, the question is how much? Ole Miss' ball hawking covered up some serious flaws, like a 59.7% completion percentage. Still, it's not like the secondary is going to completely crash and burn, just regress to an above average unit. They lose two starters: Senquez Golson, who had 10 of the team's 22 interceptions, and Cody Prewitt.
7 Arkansas. Could be a sneaky good unit. Their 6.9 YPA was pretty lousy as is a 123.39 opponent passer rating. But they kept the completions down and had a huge 33.18% PD/Comp ratio, even with that pass rush. The DB's were putting in work last season, and most of the unit returns.
8 Kentucky. AJ Stamps keys a decent unit. They pretty much hover around league average in every skill, but don't do anything exceptionally poorly or well. They won't embarrass you and they won't get embarrassed.
9 Mizzou. Mizzou allowed over a 60% completion rate yet also allowed only 6.3 YPA. I can't really square those numbers. They also had fairly poor PD/Att and PD/Comp numbers, suggesting any success the pass defense had was due to the pass rush, not the secondary. We'll see.
10 Auburn. The Tigers are a lot like Ole Miss, only their unsustainably high interception rate failed to cover up the flaws elsewhere on the defense, as the Tigers ranked 12th in yards/game and yards/attempt. The Tigers do return Jonathan Jones, who tied for the team lead with six picks and twelve PBU. No other returning defensive back had more than 2 PBU's last season, though Ford had 3 picks to go with his 1 PBU.
11 Mississippi St. No one gave up more passing yards than the Bulldogs, but no one faced more pass attempts either. Their rate stats were actually pretty decent, and their PD/Comp ratio was among the SEC's best at 33.71%. The unit was likely better than their raw numbers, but unfortunately, only one starter (Will Redmond) returns. They will almost start from scratch this year.
12 South Carolina. Man, this unit was awful. They allowed a passer rating over 130, a completion percentage of 62.1%, and a YPA of 7.3. I'm not sure if returning three starters is a positive or not.
13 Texas A&M. The Aggies were in the basement for PD/Comp and PD/ATT (20.25% and 11.37%, respectively), but at least they also had a ridiculously low INT/Pd rate, so they can expect a bit of a bounce back there. Good thing, too, as they allowed 19 TD to only 5 INT. Just a cover your eyes awful unit last year with little help on the way. Time to just play the kids and pray.
14 Vanderbilt. Vandy allowed a 62.1% completion rate for 7.6 YPA. I can't even fathom how awful those numbers are. Most of the unit returns, but I'm not sure why you would want them to. This unit got lit up last year, and will likely get lit up this year.
Poseur and I are more or less in lock-step at the top here, and there is definitely a really steep drop after that.
1 LSU. The league's most efficient pass D a year ago with three returning starters and a bevy of talented reserves. If they can just find a way to get their hands on a few more passes this unit will be the best in the country.
2 Florida. Vernon Hargreaves has been great from the moment he stepped on campus, and there's a lot to like with guys like Jalen Tabor, Brian Poole and Marcus Maye. Question is, does their inconsistency shine through a little more with a potentially weakened pass rush sans Dante Fowler Jr.?
3 Georgia. This group may not have the talent of the top two secondaries, but they function well within their unit, and they don't give up big plays.
4 Tennessee. Cam Sutton broke out nicely last season at corner, and senior Brian Randolph is somebody I would watch to maybe have a huge senior year.
5 Ole Miss. Losing Senquez Golson is no small thing, but Tony Connor has a case for the league's top safety spot and he's flanked by two seniors that operate really well in this defense in Trae Elston and Chief Brown.
6 Alabama. Bit of a downward trend going on here with Alabama. They struggled at corner but were pretty good at safety last year, and now the two safeties are gone. Some young players need to mature quickly. I don't buy the Cyrus Jones hype.
7 Kentucky. This was a solid unit, with three starters back and one pretty good safety in A.J. Stamps. I do wonder if we see them take a hit without Bud Dupree chasing quarterbacks.
8 Missouri. Kind of the same deal as Kentucky. Experience back. Not an awful group. But could be exposed if the pass-rush drops off.
9 Arkansas. Physical, veteran group but not one that overwhelms.
10 Auburn. The best thing Auburn had going for them was that they intercepted a ton of passes last year (and the fact that their pass defense was awful despite that is a testament to the randomness of turnovers, and the value of play-to-play efficiency). But 10 of those picks came from players not on the roster, and again...this really wasn't an efficient group. And now they have a coordinator that is known for asking a lot of his defensive backs.
11 Mississippi State. Really hit hard by graduation. Will Redmond should be fine at corner, but otherwise? And this wasn't that strong of a unit last year anyway.
12 South Carolina. All that talk about secondaries exposed by a lack of pass-rush? That was South Carolina last year. Maybe the three returning starters have grown from it, but I'll have to see that first.
13 Texas A&M. Another case where returning starters aren't necessarily a good thing. Intercepted just FIVE passes last season.
14 Vanderbilt. Poor Vandy.
1 LSU. Undisputed champion here. LSU allowed the fewest YPG, lowest rating, lowest YPA, fewest passing TDs and lowest completion percentage in the league, and returns three key members from the 2014 team, including burgeoning superstar Jamal Adams. The second CB spot is still open for grabs, but there's so much talent there, I can't find myself getting too worried.
2 Georgia. They return 3 starters and were right behind LSU in most every category last season. There's not a standout star in the unit, just a bunch of awfully good players that play well together.
3 Florida. I'm tempted to drop this unit solely because I think Muschamp is a major reason for their success, but there's just too much talent to ignore. Obviously Hargreaves is the man, but Tabor and others make them as loaded as any team in the country, except maybe LSU.
4 Tennessee. I know we're ranking the units, and UT's is certainly strong, but I think they'll especially benefit from what should be a good pass rush. If Tennessee can find any sort of answer for stopping the run, they might just have a defense on their hands.
5 Alabama. Okay, this is primarily talent based. They struggled at times last season, but I think Poseur is right that their overall numbers suffer mostly due to playing with large leads. It's been a bit of a weak area, comparatively, in recent seasons, but surely the slew of 5 stars will eventually put it all together.
6 Ole Miss. I want to drop them further, but this is a tough group for me to rank. I think Prewitt and Golson are huge losses, but I also like Connor, Elston, Hampton and even Shepard. This should be a good unit.
7 Auburn. I'm gonna call this the Muschamp bump. Just like I wanted to penalize Florida for losing him, I'm crediting Auburn for gaining him. I know, I know, they are pretty rough around the edges, but I have that much faith in Boom.
8 Arkansas. Last year's numbers weren't great, but they do return a lot. Couple that with their grind it out offense and it could boost their numbers on the surface.
9 Missouri. Their YPA was strong, but I think losing their pass rush will put a dent in those numbers.
10 Mississippi State. Raw numbers look rough, but rate stats were pretty solid. That said, like most of the rest of their team, they are replacing so much it will be difficult to replicate the success.
11 Kentucky. Their numbers weren't bad, but they also don't stand out to me. This unit could be a few spots higher, but they don't get me excited much.
12 Texas A&M. They stink.
13 South Carolina. They stink worse.
14 Vanderbilt. Somehow, they stink worst of all.
Almost complete consensus. We all voted for the same exact top 5 and in nearly the same order, except for Billy having Ole Miss as 5th instead of Bama. Paul, again, rates Auburn higher than I do though I find support from Billy this time, who is our unofficial Auburn swing vote. To give Paul credit, I at least see his argument here: the middle of the pack is pretty soft, Muschamp is a great coach, and he's working with fresh clay. It wouldn't take a massive leap to get to 6th in the conference. I think he's wrong, as I think if any Auburn unit is going to take a massive jump, it's the D-line, but the argument isn't crazy pants.
And as long as we're talking improve to be middle of the pack instead of somehow magically having the best defense on earth, as many national outlets seem to believe, then I largely agree with that argument.
6 Ole Miss
11 Mississippi St
12 South Carolina
13 Texas A&M
14 Vandy (of course)