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October 24, 2004

The Most Dominant NFL Defense of the Past 20 Years?

They didn't win the Super Bowl, But the '91 Eagles played
some of the best defense in modern NFL history

Despite the fact that the team that I root for has never won a
Super Bowl, I have always taken solace in the fact that my team's
brand over the past 20 years has, for the most part, been defined
by its defense. Though rarely blessed with a high-flying offense,
the Philadelphia Eagles have managed -- through multiple coaching
and ownership changes -- to maintain a reputation as a team that
plays tough, attacking defense. And there is a unique pleasure in
rooting for a defensive team: you know that, even if your guys
don't score very easily, you'll at least be able to watch the other
guys get socked in the nose. You might beat our guys, but at
least our guys will beat you up a bit.

Although the recent Jim Johnson era has certainly produced some
exciting defensive teams for the Birds, the defining Eagles defense
was the 1991 team. With no fewer than five starters -- Eric Allen,
Jerome Brown, Clyde Simmons, Seth Joyner, and Reggie White --
making the Pro Bowl, this team was the culmination of the Buddy
Ryan era (though Ryan was fired the previous offseason) and the
most dominant Eagles defense of the past 20 years.
Unfortunately, the offense, particularly the QB position, was
shattered by injuries that year and did not even make the playoffs
(though they managed a respectable 10-6 record). But how
good was that defense? Do they deserve to be mentioned
among the best NFL defenses of the past 20 years?

Any attempt to compare teams or players from different years (let
alone eras) is fraught with unresolvable hypotheticals and
mind-bending what-ifs. To attempt any sort of statistical analysis
across years is to venture down a rabbit hole of barstool bluster.
Still, given that one offers the proper disclaimers and warranties,
some effort can be made to make comparisons across years and
teams, and I NEEDED to know how the '91 Birds stacked up. So
here we go:

In approaching the question of which was the most dominant NFL
defense of the past 20 seasons, this analysis has taken the
perspective of comparing teams to their peers from each season
as opposed to attempting to compare teams across years. That
is, the analysis focuses on how a few select teams (the 1985
Chicago Bears, the 2000 Baltimore Ravens, the 2002 Tampa Bay
Buccaneers and the 1991 Eagles -- those generally acknowledged
as the most dominant defensive teams of the past 20 years)
compared to the rest of the league in the year in question. The
rationale for this distinction is that due to expansion, fluctuations
in league-wide talent, officiating trends, etc etc., it's tough to
compare the yardage surrendered by the '85 Bears to that given
up by the '02 Buccaneers. Thus, my research methodology has
focused on comparing each of these teams to their peers in the
season in question.

Specifically, I've sought to quantify how each of these teams
measured up against league-wide averages in four categories:
rushing yards allowed, passing yards allowed, total yards allowed,
and total points allowed. The analysis takes each of these team's
performance and compares it to the league average for the year,
and assigns the performance a score (essentially, just a z-score for
those inclined to be inquisitive about such things) based upon
how many standard deviations the team's performance sits above
the mean. I then compare the scores among the four teams to
see how they stack up. This is admittedly a simplistic way to
approach the problem (omits other potentially relevant categories,
does not adjust for strength of opposition, etc.), but I think it
does offer us some insight into just how good each of these
teams actually was.

Rushing Yards Allowed

Certainly all strong defenses are built upon a foundation of
punishing run defense, and these four teams are certainly no
exception. All save for the 2002 Bucs led the league in rushing
defense:

Of particular note is the staggering 970 rushing yards surrendered
by the Ravens in 2000 -- below the 1,000-yard mark, and nearly
half the league average. Though, when we factor in the standard
deviations, this figure is normalized somewhat:

Here we see (with z-score, or number of standard deviations
above the mean, given on the y-axis) that although the Ravens
were indeed the most dominant run defense of the four teams
sampled, when compared to their peers, they weren't that far
ahead of the Eagles and Bears.

Passing Yards Allowed

Passing defense is a tricky category, in that teams with strong
offenses will often force teams into throwing quite a bit in the
second half of games. Thus, only two of these four teams ranked
number 1 in the league against the pass. Still, while the '02 Bucs
were not as strong as the other three teams in rushing defense,
their pass defense was especially strong:

Normalizing the data reveals that the Tampa pass defense was
especially strong when compared to these teams:

Thus, only the Eagles could favorably compare to the Bucs -- the
Ravens and the Bears were somewhat pedestrian in this category,
both fewer than one standard deviation better than the mean.

Total Yards Allowed

Although the total yardage category is just the addition of the
rushing and passing categories, it gives a good sense for the
all-around strength of a defense. Perhaps unremarkably, all four of
these teams were number one in the NFL in total yards allowed:

What is remarkable is just how high the z-scores in this category
are -- the lowest being just below 2 standard deviations, and the
highest being just below 3 standard deviations above the league
mean:

That the Eagles were nearly three standard deviations above the
mean is actually pretty staggering. What makes it even more
notable is that the total yardage numbers around the league were
actually much lower in 1991 than in any of the other years
measured. In fact, the 1991 Eagles surrendered the fewest total
net yards of any team of the past 20 years, and are on a very
short list (three teams) who held opponents under 4,000 yards.
Pretty impressive for a team that didn't make the playoffs.

Scoring Defense

Yardage totals aside, the most telling aspect of a team's defensive
prowess is the statistic that is most important: points allowed.
Yards surrendered do not go on scoreboards, and they don't
determine the winners of games. Admittedly, there have been
teams that have been excluded from this analysis that have led
the league in points surrendered but have not led in the yardage
categories as well (all four teams included in this analysis led the
league in three of the four categories examined). Yet since 1982
only four clubs have held opponents to under 200 points -- the
'85 and '86 Bears, the '00 Ravens and the '02 Bucs:

Certainly all of these numbers are impressive, but none more so
than the Buccaneers -- if only because overall scoring was higher in
2002 than in any other year measured here. This is reflected in
the respective z-scores:

It is here that the distinction becomes most clear between the
successful (Super Bowl Champion) teams and the also-rans
becomes most clear. Though the Eagles ranked fifth in the
league in points surrendered, they were only one standard
deviation above the mean. One can imagine a number of reasons
why this is the case, especially in the context of the other data:
offensive turnovers leading to short fields for opposing offenses,
the inability of the Eagles offense to hold the ball, lack of
turnovers, etc. But this single stat seems to be what separates
the Super Bowl Champions from the 1991 Eagles.

Conclusions

So the 1991 Eagles defense probably wasn't the best NFL
defense of the past twenty years -- but it's pretty close. The
killer is that they just didn't do a strong enough job of keeping
points off the board -- at least not compared to the teams that
won championships. Still, there does exist very convincing
evidence that they most definitely belong on the list of the best
defenses of the past 20 years. They had the most Pro Bowlers
(five, the same as the Bears and more than the four for the Bucs
and the three for the Ravens) and played with far and away the
worst offense of any of these teams (and that includes the
Ravens, and I'm not kidding). It's a terrible shame that this
defense was wasted on a team that, due to injury after injury,
featured the following cast of characters at quarterback: Randall
Cunningham (lasted four attempts in week one), Jim McMahon,
Jeff Kemp, Brad Goebel (0 TDs, 6 INTs), and Pat Ryan (0 TDs, 4
INTs). Pretty brutal. The real shame is that this team wouldn't
get another chance. Following the '91 season, defensive tackle
Jerome Brown died in a car crash near his home in Florida.
Though a Pro Bowler and widely respected around the league,
the squad's play in subsequent years demonstrated just how
special he was, both against the run and the pass. The team was
never really the same again.

So that was the year, and it didn't happen. The Eagles still don't
have a Super Bowl, and Philadelphia will likely never see that sort
of group again. The irony of losing to Tampa Bay in 2002-2003
(on an INT no less) should be lost on no one in Philadelphia, since
that Tampa Bay team was the closest the NFL has seen to the
Eagles pass defense in the past ten years. And so the Birds soldier
on, and so do the fans, hoping that some day someone will
release a 1991 Eagles Defense Highlight video. Until then, we'll
have to take some comfort in the numbers, and the fact that
while other teams may have beaten our guys, our guys definitely
beat up on the rest of the NFL that year.

Author's Note: if anyone has any better ideas for how to handle
this data, or any additional strategies for thinking about it,
definitely get in touch at the e-mail address below:
thatkid@thatkidinthecorner.com.

Posted by thatkid at October 24, 2004 5:24 PM under Sports

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