The 2013 Seattle Seahawks rode one of the best single-season defenses in history to a Super Bowl romp over Peyton Manning and the Denver Broncos. They were recognized at the time for being one of the best defenses ever, but a common critique was that it was just one season. The Chicago Bears in the 80s, the Baltimore Ravens at the turn of the century, the Steel Curtain Steelers of the 70s, and others, had sustained dominance. Obscured by an injury-ravaged finale of 2014, the Seahawks defense has now quietly put together a three-year stretch of dominance never before seen in the NFL.

The process

Every defensive statistic tracked at Pro-Football-Reference.com was compiled and summarized over a three-year period from 2012-2014. The same was done for six of the most dominant defenses in history:
  • 1984-1986 Chicago Bears
  • 1969-1971 Minnesota Vikings
  • 1999-2001 Baltimore Ravens
  • 1974-1976 Pittsburgh Steelers
  • 2001-2003 Tampa Bay Buccaneers
  • 1990-1992 Philadelphia Eagles
The years selected for each defense represented their best three-year stretch, so as to put their best foot forward. Philadelphia made the list because FootballOutsiders.com lists the 1991 Eagles defense as the best single-season defense ever. The 2002 Bucs ranked highly both on the DVOA rankings and my own. Minnesota’s 1969 Purple People Eaters were the best single-season defense I measured. The other combatants should need no explanation.
Points allowed and yards allowed were selected to compare the defenses. There are a variety of ways to define success for a defense, but points and yards are the generally accepted yard sticks, so they were the focus.

Round 1: Absolute numbers

There are obvious problems with comparing defenses from different eras using absolute numbers. Teams played fewer games in the early 70s. There were fewer teams. Rules have changed drastically, and favor the offenses far more now. The size and the speed of athletes has increased dramatically. Still, it is helpful to at least lay the foundation in absolute terms before getting into more contextual measures.

Absolute Points Per Game Over Three Years

Rank Years Team PPG
1 1969-1971 Minnesota Vikings 9.9
2 1974-1976 Pittsburgh Steelers 11.6
3 1984-1986 Chicago Bears 13.2
4 1999-2001 Baltimore Ravens 14.7
5 2012-2014 Seattle Seahawks 15.2
6 2001-2003 Tampa Bay Buccaneers 15.4
7 1990-1992 Philadelphia Eagles 16.4

That Vikings defense was insane. Holding opponents under 10 points per game in any era is nuts. Pittsburgh is a few points behind, and the Bears are a few points behind the Steelers. The Ravens, Seahawks and Bucs are roughly equivalent and the Eagles bring up the rear. Now let’s look at yards allowed:

Absolute Yards Per Game Over Three Years

Rank Years Team YPG
1 1969-1971 Minnesota Vikings 212.6
2 1974-1976 Pittsburgh Steelers 239.5
3 1984-1986 Chicago Bears 252.7
4 1990-1992 Philadelphia Eagles 262.9
5 1999-2001 Baltimore Ravens 263.2
6 2001-2003 Tampa Bay Buccaneers 274.2
7 2012-2014 Seattle Seahawks 282.3

If you had not already decided this is a biased article written by a homer blogger, the thought certainly has to cross your mind after seeing the Seahawks at the bottom of this list. How else could anyone come to the conclusion that this Seahawks defense is the best ever? Hold that thought as we start to examine how these defenses performed relative to the era in which they played.

Round 2: League rankings

The assumption with all of these defenses is that they were the best in their era. Nobody was better. The truth is that most of these defenses were not even consistently ranked at the top of the league for either points or yards allowed over these three-year stretches. That kickass Ravens defense from 1999-2001? They only ranked #1 in the NFL for points allowed in one of their three measured seasons and never ranked at the top of the league in yards allowed. Being the best involves doing it better than anyone else for a consistent period of time. Some of these teams did a better job of that than others.

Average NFL Rank In Points Per Game Over Three Years

Rank Years Team Avg PPG Rank
1 2012-2014 Seattle Seahawks 1.0
2 1969-1971 Minnesota Vikings 1.0
3 1984-1986 Chicago Bears 1.7
4 1974-1976 Pittsburgh Steelers 1.7
5 1999-2001 Baltimore Ravens 3.7
6 2001-2003 Tampa Bay Buccaneers 4.3
7 1990-1992 Philadelphia Eagles 4.3

Seattle has ranked #1 in the NFL in points allowed in each of the last three seasons. The only other team since the AFL/NFL merger to accomplish that feat was the 1969-71 Vikings (Note: I’m grandfathering in the Vikings despite the streak starting before the merger).

The Seahawks have a chance to become the first team since the merger to rank #1 in points allowed for four straight seasons

All of these teams averaged a top five ranking in this category for a three-year stretch, which is fantastic. Seattle and Minnesota’s performance relative to their peers stands out.

Average NFL Rank In Yards Per Game Over Three Years

Rank Years Team Avg YPG Rank
1 1984-1986 Chicago Bears 1.0
2 1969-1971 Minnesota Vikings 1.3
3 2012-2014 Seattle Seahawks 2.0
4 1999-2001 Baltimore Ravens 2.0
5 1974-1976 Pittsburgh Steelers 2.0
6 2001-2003 Tampa Bay Buccaneers 4.0
7 1990-1992 Philadelphia Eagles 6.3

Chicago is the only team in NFL history to go three straight years ranked at the top of the league in yards allowed. Seattle has done it for two straight years, and could become just the second team to accomplish that feat should they lead the league again this season. Even if points allowed is a far more important measure of a defense, the Bears accomplishment in this area is historically unparalleled.

Round 3: Standing out from the crowd

The best way to account for all the variables impacting defensive performance across these different eras is to measure how these teams performed relative to the league averages during that time. To do this, I totalled the league averages for points allowed and yards allowed during each of these three-year stretches, and then calculated the standard deviation so we could see just how special these defenses were relative to the rest of the league.
The Steelers are one of the most well known defenses due to their nickname, popular players, and collection of Super Bowl wins. What many people do not understand is that they played in the lowest scoring era in NFL history. Their performance, relative to the rest of NFL defenses in that era was not nearly as dominant as their reputation.

Standard Deviations From League Average In Points Per Game Over Three Years

Rank Years Team Avg Def PPG Allowed PPG Std Dev
1 2012-2014 Seattle Seahawks 22.9 -2.90
2 1984-1986 Chicago Bears 21.1 -2.49
3 1969-1971 Minnesota Vikings 19.3 -2.49
4 1999-2001 Baltimore Ravens 20.6 -2.18
5 2001-2003 Tampa Bay Buccaneers 20.9 -2.09
6 1974-1976 Pittsburgh Steelers 19.0 -2.08
7 1990-1992 Philadelphia Eagles 19.3 -1.03

Offenses are scoring more than ever before. Seattle is playing in an era where teams are scoring nearly 23 ppg, which makes their 15.2 ppg allowed far more impressive than when it is examined in isolation via absolute numbers as we did in Round 1. In fact, they are lapping the competition in the most important measure covered here.

Seattle is nearly three standard deviations better preventing scoring than the rest of NFL defenses in their era. That is a mind-boggling number and significantly ahead of the rest of history’s best defenses.

A one-hit wonder team like the Eagles is exposed by this method of evaluation. They do not really belong in this list. They were not even the best performers in this category during this stretch of time. The 1990-92 New Orleans Saints were -1.78 standard deviations better than the league average. The San Francisco 49ers and Washington Redskins also bested the Eagles in this measure.

It is hard to find teams that are two standard deviations better than the league average. These are the best defenses to ever play, and three of them are right around two standard deviations better than average defenses in their era. Seattle pushing up near three, and being almost a half-a-deviation better than the nearest competitor speaks volumes about what the Seahawks are doing in a fantasy football-driven era of the NFL.

Standard Deviations From League Average In Yards Per Game Over Three Years

Rank Years Team Avg Def YPG Allowed YPG Std Dev
1 1984-1986 Chicago Bears 327.8 -3.07
2 2012-2014 Seattle Seahawks 347.9 -2.98
3 1969-1971 Minnesota Vikings 287.3 -2.45
4 1999-2001 Baltimore Ravens 318.6 -2.34
5 2001-2003 Tampa Bay Buccaneers 320.8 -2.09
6 1974-1976 Pittsburgh Steelers 297.6 -2.02
7 1990-1992 Philadelphia Eagles 304.5 -1.89

Chicago breaks the sound barrier by besting the league averaged in their era by over three standard deviations in yards allowed. But Seattle is right there with them at just a smidge below three. Both teams are far ahead of their nearest competitor, the Vikings.

Again, yards allowed is not nearly as important as points allowed, but it represents a level of dominance over an opponent that deserves recognition.

At this point, only three defenses merit consideration for the best of all-time: the Bears, Seahawks, and Vikings. The rest of these defenses were fantastic and dominant, but simply did not stand out the same way the top three did relative to their peers.

Picking the best

This is a little like picking between fresh-baked cookies, ice cream, or brownies. Each of the three finalists are terrific in their own right. The best defenses should not only suffocate opposing offenses, but do so in a way that was rare for the era in which they played. Holding opponents to under 15 ppg is impressive, but less so if three or four other defenses did the same. 
Seattle stands out in this way more than any other defense. They were essentially three standard deviations better than the average NFL defense during their time for both points and yards allowed. They dominated the most important measure of all in points allowed. To provide even more color, their 15.2 ppg is outstanding, but consider that only one other defense in the NFL (the 49ers) allowed under 20 ppg during that time. Seattle allowed nearly a third fewer points than the 3rd-best team (the Bengals at 20.2). 
In a league determined to advantage the passing game, Seattle is the only team in their era to hold opposing quarterbacks to a three-year passer rating under 70.0 (69.7), well below league average at -2.47 standard deviations. The next closest is the Bengals at 75.0 (-1.62). 
Chicago might have been my selection if not for the Seahawks dominance in points allowed relative to their era. If they were close, the Bears would have earned the title based on their slight lead in yards allowed relative to era and consistently solid rankings in each measure. Knowing just how different the game is now compared to then, and how much more dominant the Seahawks have been even with those rule changes, swung the decision in Seattle’s favor.

The Seahawks will now set their sights on a fourth straight season of excellence. There is reason to believe this could be the best defense they have fielded yet given their young depth on the defensive line and maturation of their linebacking corps. Another dominant season, another trip to the Super Bowl, and another ring would put an exclamation point on what has already been a historically great run of defense.

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