Saturday, June 29, 2013

The Measure Of Defensive Greatness: Best Scoring Defenses In NFL History

Defining greatness is subjective. Different people value and emphasize different things. There have been some defensive squads in the NFL whose greatness was undeniable. For those titans of the gridiron, the debate becomes which were better, and how do defenses of one era compare to those of another? This series will attempt to shed light on those questions while also exploring how far this young and talented Seahawks defense is from joining the ranks of the truly elite.


Part I: Scoring Defense

The Purple People Eaters of the early 70s Minnesota Vikings were bad dudes

The Point of Points

There have been great secondaries, great defensive lines, and prolific thieves that collected opponent turnovers by the bushel. Every defense is ultimately measured by their ability to keep opponents off the scoreboard. Opponent scoring average is a great indicator, even if there are some flaws. Points scored on offensive turnovers or in special teams show up on opponents scoring average and have nothing to do with the defense. A terrible offense can put the defense in precarious situations where even a three-and-out series results in a field goal. Still, legendary defenses find a way to keep their opponents scoring down. The game has changed too much to rely solely on points allowed as the measure of a great scoring defense. We will explore some more nuanced approaches to figuring out which defenses were the best ever at keeping opponents off the scoreboard.

Note: Due to a variety of factors, the research for this series will only go back to 1960. If time allows, I will update with data from the 30s, 40s and 50s as well.

Start with scoring 

Despite the flaws, it makes sense to begin the search for the best scoring defense in NFL history by the amount of points allowed. NFL schedules changed over time, so absolute points allowed tells us less than points per game.

Source: Pro-Football-Reference.com
It comes as no surprise that the legendary defenses of the Pittsburgh Steelers from the 70s, the Minnesota Vikings from the late 60s and early 70s, the Chicago Bears of the 80s and more recent stalwarts from Baltimore and Tampa show up among the Top 30 scoring defense of all-time. But how do you compare defenses that played in such different eras? 

Injecting offense into defensive evaluations

Pro-Football-Reference.com has a nice feature that allows us to see how many points teams scored, on average, per game in any given season. I expected to find a gradual increase in scoring over the years. I was wrong.

The 70s seemed to have great defenses and suspect offenses

There was plenty of scoring going on during the 60s. It has only been in the last few years that teams have approached the record set in 1961 of 23.1 points per game. There was a huge trough in the 70s, and again in the early 90s. Evaluating a great scoring defense without factoring in the state of scoring in the league that year would be incomplete, at best. 

One way to accomplish that is to figure out how varied scoring was among the teams in each given season (also known as standard deviation), and then figure out how much did each defense stand out from the norm. The following table lists those same Top 30 scoring defenses, except now the amount of points offenses were scoring per game that season is factored in. It is sorted by how many standard deviations the opponent scoring average was compared to the average amount of points teams were scoring in the league that season. The standard deviation was calculated by looking at the average points per game scored by each team in each season. This helps tell us how varied the offensive output was in that year. The smaller the standard deviation, the tighter the bunching of what offenses were averaging that year. 


No team better illustrates the importance of contextualizing defensive performance than the 1977 Atlanta Falcons. They own the bragging rights as the best scoring defense of the bunch, allowing only 9.2 points/game. Their accomplishment is less impressive when factoring in a historic low in scoring across the league that year. The average team scored 17.2 points.game. The Falcons were just about two standard deviations below the norm. That still registers as one of the great defensive seasons in history, but drops them 19 spots when comparing greatness to greatness. Other teams hurt by an overall lull in offense across the league were the vaunted 1976 Steelers, who dropped 14 spots, and the two other 1977 teams that show up on the list, the Los Angeles Rams (-17 spots) and Denver Broncos (-16 spots). 

Teams whose accomplishments were amplified included some modern era teams like the 2002 Bucs (+17) and the 1985 Bears (+16). Both jumped into the Top 10. Teams averaged more than four points more per game in 2002 than in 1977, and the Bucs allowed only three points more per game than the 1977 Falcons. Tampa also benefited in the rankings by a smaller standard deviation, meaning that more teams were bunched around that average score per game and that they were less likely to face a team with an offense far off the average. 

Other teams barely flinched when offensive output was factored in. The 1969 Vikings moved up one spot to the top. The 1971 Vikings moved up one spot as well, as did the 1971 Baltimore Colts and the 1973 Miami Dolphins.

Note that the first table listing absolute scoring defense leaders had six teams from the 70s, and only one team from after 1977 in the Top 10. The new table has a more varied Top 10 that includes three teams from the 60s, four from the 70s and three from the 80s and later.

Defensive Scoring Efficiency

There are other variables to consider when evaluating scoring defenses across the decades. Any Seahawks fan that watched the 1992 season can tell you the impact of terrible offense on a great defense. The best measure here would be how many points does a defense allow per possession, combined with looking at their effectiveness in the first seven or eight opponent possessions. An offense that keeps putting their defense on the field puts strain and wear on that defense that cannot be measured by absolute numbers. There do not appear to be statistics about possessions that date back far enough to drill-down there. Instead, we can look at the number of opponent plays and the amount of points per play these defenses allowed. A defense can be at fault for yielding more plays, so there are some flaws here, but it is still instructive.


We are back to seeing a 70s-dominated Top 10, but there are still some insights to be gained. The Dolphins defenses drop to the bottom of the list. they were barely on the field in those two seasons, helped by an offense that finished 4th in scoring in 1971 and 1st in scoring in 1972. It is also interesting when comparing two teams of the same era. The 1985 Bears defense is one of the most famous in history, but the 1986 Bears defense was better in every way opponent scoring has been analyzed. Here, we see the 1986 team was on the field for far more plays, but still managed to allow far fewer points per play. 

Championships

That 1985 Bears defense will always be the one people remember because they ended the season with a Super Bowl ring. At each point in this series, we will examine the relationship of various types of defensive strength to championships. In this case, 16 of the Top 30 scoring defenses in history made the Championship game (not always the Super Bowl), with 12 coming away with a ring. 

The 2012 Seattle Seahawks

Seattle led the NFL in scoring defense last season at 15.3 points/game. That was among the highest totals to lead the league in a given season. Again, context matters. The only time offenses averaged more points in a season than the 22.8 points/game they scored in 2012 was in 1965 when they averaged 23.1 in 1965. The top scoring defense in that season belonged to the Green Bay Packers, who allowed 16.0 points/game. 

They allowed 0.253 points per play, which does not place in this list of great defenses. There is plenty of improvement needed there to reach the next plateau.

The 2012 Seahawks defense was only 1.6 standard deviations below the league scoring average. That puts them in decent company among the Top 30 from this sample, but not exactly elite. It is going to be hard for any defense to be historically significant without holding opponents to less than 13.0 points per game. Either that, or offenses are going to have to continue their upward climb to where teams are averaging 24 or 25 points per game. 

Either way, this Seahawks defense has some ground to make up in the scoring category if they want to become one of the best in history, as opposed to just the best in any given season.

The Best Scoring Defense In History

Carl Eller, and the rest of the 1969 Purple People Eaters in Minnesota earn the title of best scoring defense in history. They finished top overall in relative scoring defense (accounting for offensive output that season), second in absolute points allowed per game at only 9.5, and were a very respectable 6th in points allowed per play. They topped it off with an NFL Championship,  before losing to the Chiefs in the Super Bowl. There may never be another team to put those kinds of defensive scoring credentials together again.

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