Head of the LOB.
Congratulations to the Seattle Seahawks and their fans on winning Super Bowl XLVIII
. With the win, Seattle has confirmed its status as one of the greatest defenses in NFL history. The Seahawks defense produced a game for the ages on Sunday: facing Peyton Manning
, Demaryius Thomas
, and one of the greatest offenses ever, Seattle’s defense
outscored Denver’s offense, 9-8. Led by Malcolm Smith
, Cliff Avril
, Kam Chancellor
, Earl Thomas
and Richard Sherman
, the Seahawks stamped their claim with the ’85 Bears, ’00 Ravens, and ’02 Bucs as one of the greatest defenses of the last 30 years.
But today, I want to look at which defenses were the best in regular season history, and see where Seattle stacks up. Bill Barnwell had an interesting post during Super Bowl week. He used the statistical measurement known as the Z-Score to show that Seattle was the tenth best defensive scoring team in NFL history. Don’t be too confused by the idea of a Z-score: in English, this just means that Seattle’s defense — and yes, I am going to conflate the concepts of defense and points allowed throughout this post — was 2.2 standard deviations above average in points allowed, one of just ten teams to ever produce such a result.
So how do we get there? Well, Seattle allowed 14.4 points per game during the regular season. The league average was 23.4 points, which means that Seattle’s defense was 9.0 PPG better than average. The standard deviation of points per game among the 32 NFL defenses in 2013 was 4.08 points per game; therefore, Seattle has a Z-score of 2.20, because the Seahawks allowed points at a rate that was 2.20 standard deviations better than the mean.
Today, I wanted to do the same analysis but adjust for strength of schedule, by deriving offensive and defensive SRS grades. Of course, Pro-Football-Reference has published offensive and defensive SRS grades for awhile, but I decided to crunch the numbers on my own and see if they matched up with what Neil and Mike did (they did). For the uninitiated, SRS stands for Simple Rating System, which is simple to understand but a bit complicated to derive. The SRS is simply margin of victory (or, in the case of offenses and defenses, margin of production above league average) adjusted for strength of schedule. The key is using an iterative process, where, in Excel, we adjust the ratings hundreds of times; after all, to adjust for SOS, you have to adjust for the SOS of each opponent, and the SOS of each opponent’s opponent, and so on.
The table below shows the top 200 scoring defenses since 1932. Here’s how to read the 2002 Bucs line. That season, Tampa Bay allowed 9.4 points per game less than league average. The average defense the Bucs faced — using the iterative method to derive SOS grades — was 0.4 points above average. Therefore, Tampa Bay is credited with an adjusted rating of 9.8 PPG better average. The standard deviation of defensive ratings in the NFL in 2002 was 3.45, giving the Buccaneers a Z-score of 2.83, the highest ever. The table below is fully sortable and searchable, and shows the top 200 defenses. [click to continue…]