Elite defenses have shorter windows than you think

Steve Mitchell-USA TODAY Sports

The Jacksonville Jaguars are a team built on winning games with their defense and running the ball to limit the mistakes of their quarterback. But how sustainable is relying that heavily on your defense?

To investigate this question, I used Football Outsider's passing defensive DVOA% stats from 2000 to 2017 and looked at the numbers from a few perspectives.

Been there before

  • Of the 90 data points that comprised the top five passing defenses for the 18 seasons examined, only 30 of the NFL's 32 franchises are represented.
  • The average top-five defense market share for teams that have previously been a top-five pass defense was 16.67 percent
  • Assuming all teams have an equal chance at being a top-fivedefense at the start of the season would suggest a rate of five out of 32 -- or 15.63 percent.
  • A team having "been there before" appears to be worth about a 6.4 percent difference.

Will they repeat?

  • When looking for a "repeat performance" of a top-five passing defense the year after a top-five passing defense, 60 percent of team's maximum consecutive years streak was one.
  • 30 percent of a team's maximum consecutive years streak was two years.
  • 10 percent of a team's maximum consecutive years streak was three or more years.
  • The longest stretch a team has gone as a top-five passing defense was four seasons.
  • The average maximum consecutive years streak for a top-five passing defense was 1.57 years.

Numerical "prediction"

  • A linear regression of a team's previous year pass defense DVOA% and that team's following year pass defense DVOA% had a correlation (r) of .342.
  • This level of correlation is enough for me to say that the relationship between previous and subsequent years of pass defenses "matters".
  • Using the equation of the line of best fit as a guide, our -27.5% DVOA% might be followed up by a -9.50 percent pass defense DVOA%
  • A -9.50% pass defense DVOA% or worse has only made 11.11% of top-five pass defenses in the tested time span.

We all recognize that defenses and teams in general have competitive windows, but I didn't realize just how short the windows are for a team that wins most of it's games due to an unreasonably healthy defense playing at such a high level.

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