Final Defensive Efficiency Rankings
This is the fourth post in my series discussing Defensive Efficiency Scores (D-SCORE). Here’s the first three parts if you missed them:
As with my offensive efficiency scores, I used a series of regression analyses to determine the weighting coefficients of the five variables I discussed. I then took those coefficients and multiplied a team’s performance relative to league average in each variable category. The final Defensive Efficiency Score (D-SCORE) is the sum of the five weighted variables. To summarize each variable:
Points Per Drive -The most important variable – it gives us a good idea of how good a defense is. However, as I noted in my offensive efficiency discussion, PPD is not a complete picture. Which of these defenses is better:
8 drives allowed: 8 FG’s = 3 PPD
8 drives allowed: 4 TD’s allowed, 2 punts, 2 INT’s in your opponents’ red zone = 3.5 PPD
In the first scenario, the defense allows fewer points. But their offense is likely to start all 8 of their offensive drives deep in their own territory. In the second scenario, the defense allows a few more points. However, by by forcing 2 punts and 2 INT’s inside their opponents’ 20 yard line, their offense is more likely to score points and thus the “extra” points they allow are largely (or entirely) negated. Now, these are obviously extreme and unlikely scenarios but the idea is that a team’s offensive and defensive outputs are related and the true value of a defense is the effect it has on the margin of victory.
For the purposes of these rankings, I took the “Adjusted” PPD – this factors in defensive points scored (safeties, fumble recovery and interception TD’s).
Touchdown% – Obvious point: preventing touchdowns is the most important thing a defense can do to limit the number of points they give up. The ability to frequently limit your opponent to field goals is a very valuable trait – one which every elite defense has in common. Think of it this way: the average field goal attempt allowed is worth ~2.3 points (between 2000-2010, the average FG% was 78%, which multiplied by 3 points = 2.3). The average TD is worth about 7 points (the rare extra point miss is made up for by the occasional 2 point conversion). Theoretically, a defense could give up 3 FG attempts and stay within a one touchdown point differential.
Yards Per Point Allowed – “Bend but don’t break”, it’s a somewhat useful measure of how “tough” it is to score on a defense. Elite defenses typically score very highly in YPP. It’s most useful in providing context to a team’s yards allowed. Teams whose defense look bad based on yardage stats fall into two main categories: teams who amass big leads and are willing to give up yards, but not necessarily touchdowns in exchange for time off the clock (e.g. early 1990’s Bills, current-era Patriots) and teams who are just awful defensively. The former will score well in YPP despite having ugly Yards Per Game or Yards Per Drive. The latter will typically score very poorly in YPP (and pretty much every other category).
Yards Per Drive – Only very minimal consideration is given to YPD. There have been a number of defenses which give up lots of yards but don’t allow a lot of points. Because preventing points – not yards – is the goal for a defense, yards allowed are fairly irrelevant to a discussion of defensive efficiency. However, we cannot ignore YPD completely because there are some implications for defenses which give up tons of yards:
More yards = more potential scoring opportunities. In theory, a defense which gives up very few YPD is less likely to give up a last minute (or overtime) game-losing FG than a defense which gives up tons of YPD.
Effect on field position – Giving up tons of yards might not lead to lots of points allowed, but there is reason to think that it can limit a team’s offensive output. In other words, teams which give up fewer YPD are less likely to get pinned deep in their own end of the field because their opponent will be gaining fewer yards and thus punting from a less advantageous (for the punting team) part of the field. A defense which gives up lots of YPD is hampering their offense’s ability to score quickly (or at all)
Time of possession – More yards often means more plays which means more time off the clock – another way to hamper your own offense’s ability to score. If your favorite team is tied with 5 minutes to go, and their opponent has the ball, which defense would you prefer – the one which gives up only 21 yards per drive, or the one that gives up 35 YPD? The former is more likely than the latter to get off the field quickly enough to give your offense time to get a game winning score.
Turnover% – Forcing a turnover is the best possible outcome for a defense. Not only does it satisfy the primary goal (preventing points) but it adds a secondary benefit (scoring points directly or indirectly). Often, teams who score well in YPP have a high TO%. While a high TO% doesn’t necessarily mean a great defense, it’s rare to see a truly bad defense which creates a lot of turnovers. At worst, a team with a high TO% will be about league average in points allowed.
I weighted the variables in this order:
Points Per Drive
Yards Per Point
Yards Per Drive
With all of that in mind, here are the 20 best and worst defenses since the 1970 AFL/NFL merger:
|7||1975||Los Angeles Rams||44.59|
|9||2002||Tampa Bay Buccaneers||44.12|
|16||1991||New Orleans Saints||38.83|
|18||2003||New England Patriots||37.55|
|1171||1999||San Francisco 49ers||-37.76|
|1172||1986||Tampa Bay Buccaneers||-38.23|
|1186||1980||New Orleans Saints||-47.57|
|1187||1975||New York Jets||-49.41|
|1188||1972||New England Patriots||-50.39|
You can all the scores here: Defensive Efficiency Scores