Sunday, January 15, 2012

The NFL's "Worst" Pass Defenses

In tonight's 45-7 drubbing of the Broncos by the Patriots, I have to admit that the 2nd half was pretty boring.

Here's what I did see that was interesting:

CBS put up a graphic showing that the best teams in the NFL this year also had the worst pass defenses. This wasn't the first time I had seen this stat trotted out as "proof" that the Packers, Saints and Pats have horrible pass defenses.

WonLostPass Yards
Allowed Per Game
NFL Rank
Green Bay Packers
151301.2 32nd 
New England Patriots
13 3 283.0 31st
New Orleans Saints
11 5 266.7 30th

Well ... duh.

Teams that win most of their games are often playing with the lead. That means that their opponents are playing from behind. Teams trying to catch up have to pass more. And teams that pass more get more pass yards.

Furthermore, these three teams have the three best QBs in the NFL. With Brady, Brees and Rodgers leading your passing attack, it doesn't take very long to score. And teams that pass use up less time on the clock. That gives opponents of these teams even more opportunity to rack up passing yards.

The problem with Passing Yards is that it's an absolute stat, not a rate stat. So circumstances that cause a team to pass more will make it look like they are "better" passing teams.

To show that good teams allow more passing yards, I took the free version of Football Mogul and created a universe where every team had the exact same players on defense. And then I used the "Simulator Tool" to simulate the 2011-2012 one hundred times, to look at the impact that a team's offense has on their passing yards allowed:

WinsPass Yards Allowed Per Game
Great Teams
13 - 16 259.7
Good Teams
10 - 12 257.2
Average Teams
7 - 9 244.3
Bad Teams
4 - 6 240.1
Horrible Teams
0 - 3 241.6

As you can see, the better teams allow more passing yards. Using passing yards allowed to measure a football defense is like using double plays to measure a baseball defense. Counterintuitively, bad teams turn more double plays, simply because they allow more base runners, and you need someone on base to turn a double play.

If the talking heads really wanted the truth, they would show this:

WonLostOpponent Passer Rating NFL Rank
Green Bay Packers
15182.2 11th
New England Patriots
13 3 84.7 16th
New Orleans Saints
11 5 88.1 24th

Alas, saying that the Patriots have an "average passing defense" doesn't make for eye-catching on-screen graphics. So for now we'll have to continue to hear about New England's incompetent secondary.

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