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The Best Defense of All Time: Methodology Discussion

Posted by Chase Stuart on December 17, 2008

In case you haven't noticed, the 2008 Pittsburgh Steelers have one of the greatest defenses in regular season history. Pittsburgh ranks 1st in points allowed, yards allowed, yards per pass allowed, passing yards allowed, yards per rush allowed, and second in rushing yards allowed, sacks, rushing touchdowns allowed and third in passing touchdowns allowed. That's an incredibly balanced and terrific defense.

Believe it or not, the team that leads the league in points allowed usually isn't the leader in yards allowed; only four teams in the last 20 years have led the league in both categories -- the '06 Ravens, the '04 Steelers, the '02 Bucs and the '96 Packers. The '85 and '86 Bears, the '81 Eagles, the '79 Bucs, the '76 Steelers, the '72 Dolphins and the '70 Vikings are the only other post-merger teams to pull off this double double.

So what's the best way to rank the defenses? Let's run down the major statistics people use to rank the defenses.

Points Allowed: People who use this stat like to say things like, "at the end of the day, all I care about is how many points a defense allowed." While that's a good statement, points allowed also includes return touchdowns. That's a big problem -- if your QB throws a pick-six or your special teams allowed a score, that shouldn't hurt the defense. Further, this is heavily influenced by opponent's field position and opponent's time of possession, two factors that don't impact all defenses equally. But many view this as the mother of all defensive stats, so it's here to stay.

Yards Allowed: This is how the NFL officially ranks the defenses. Yards allowed avoids the problem of return touchdowns and opponent's field position, but it's a far from perfect statistic. Not all yards are equal, and teams that face a lot of pass attempts (which is not a sign of a bad defense) are prone to giving up lots of yards. Combining passing yards and rushing yards is Just Plain Wrong. But it's not as bad as...

Yards per Play Allowed: Every once in awhile, you'll hear analysts or read writers using a stat called yards per play (or for defenses, yards per play allowed). This is an awful stat that should be deleted from our lexicon; it's subject to Simpson's Paradox; for example, Clinton Portis is averaging more yards per rush and more yards per reception than Warrick Dunn, but fewer yards per play (or touch) than Dunn. Yards per play allowed is even worse than yards per play; let me explain why.

I've often wondered whether offense is simply the flip side of defense. I think a great offense will score about as many points against a great defense as an average offense will score against an average defense or a bad offense will score against a bad defense. Great offenses can lead teams to Super Bowls ('06 Colts, '99 Rams, '98 Broncos, any of Joe Montana's teams) just like great defenses can lead teams to Super Bowls ('02 Bucs, '00 Ravens, '85 Bears, any of the '70s Steelers teams). But there is at least one big difference between offense and defense. If you have a great passing offense and a bad rushing offense, you can be a great offense; if you have a bad passing offense and a great rushing offense, you can be a great offense. If you have a bad unit on defense, you can't be a great defense. Unlike offense, a defense is like a chain -- it is only as strong as its weakest link.

If a defense is dynamite against the run but terrible against the pass, teams will simply pass on them all day. On offense, you can easily hide a bad unit; on defense, there is nothing you can do. An offensive coordinator wants to be efficient; a defensive coordinator wants to be balanced.

In that way, if you measure yards per play correctly -- taking total yards, subtracting 50 points for fumbles lost, removing all sack yards, subtracting 45 yards for interceptions, and then dividing that number by pass attempts, rush attempts and sacks -- it's a decent measuring stick. But it still won't work for defense, where one number won't tell the story. A defense that's 10th in both passing and rushing defense (measured the correct way) should be better than one that is 1st in one type of defense and 19th in the other. Therefore, we need to break defenses down into their two components.

Adjusted net yards per pass allowed (ANYAA) : You knew this was coming. There is no better way to measure a pass defense than ANY/A allowed. The calculation explanation is at this link. People use passing yards allowed, or yards per pass allowed, or even just sacks to measure a pass defense, but this metric incorporates all of them.

Adjusted yards per carry allowed: This is simply rushing yards plus 20 yards for every rushing touchdown divided by the number of carries. With both ANY/AA and AYPCA, I am going to simply use the rate number. Usually when I'm looking at per attempt numbers, I'll compare that number to the league average and multiply the difference by the number of attempts; that's a great way to measure value. But sample size isn't a big issue here -- every team plays a full season -- and the "weakest link" argument comes in here, too. If a rush defense is great, it won't face that many attempts, and therefore would be undervalued. Further, if a defense is on the field all the time because the offense is bad, it's much more likely to be at the top or bottom of the rankings if you multiply the defense's rate numbers by the number of attempts. I'm comfortable sticking with averages here.

Offensive touchdowns allowed: This improves on points allowed in a few ways. One, teams aren't penalized or rewarded based on how field goal attempts go; a missed 30 yarder or a made 50 yarder are misleading indicators of a team's defensive ability. Further, it eliminates all return touchdowns. It reward teams that make goal line stands. Obviously it's not perfect -- field position still plays a big role here and allowing field goals isn't a great thing -- but I think it's a useful statistic to keep in mind.

Fumbles, Safeties and Touchdowns: A fumble is worth 50 yards. A defensive touchdown is worth 6.4 points -- after all, it gives the opponent the ball once again at about the 27 yard line. But what is a safety worth? The team that gets the safety usually gets the ball at around the 40 yard line. However, safeties usually come in situations when the team with the ball isn't likely to score next, anyway. Pretend that instead of a safety, the ball carrier is tackled at the one foot line. First and 10 from that yard marker is worth -1.6 points, and we know the team with the ball must either be facing second, third or fourth down and at least ten yards to go. I think it's safe to say that if you convert the average safety into the average "down at the one", the team with the ball is in a -2.0 situation. Thinking of it another way, the average field position of the defense following that situation is probably about the 50 yard line, which is a +2.0 situation for that team. This means the value of a safety is really just 1.6 points; the two points on the board plus the 0.4 point loss in field position following the safety (moving from the 50 to the 40). In a vacuum, a safety is really valuable because it's two points and the ball; but in a vacuum, tackling the ball carrier at his one is really valuable too, because it means you're probably going to get the ball really soon in really good shape. So net, a safety is worth only about one-fourth as much as a defensive touchdown. If we convert fumbles to yards -- and 12-13 yards is worth about one point -- a fumble is worth 4.0 points. That makes five defensive touchdowns equal to eight fumble recoveries equal to 20 safeties. Does that seem right to you? I'm going to hold off on this category for a little bit and see some of the responses here.

That leaves me with two scoring (Points Allowed and Offensive Touchdowns Allowed) and two yardage (ANYAA and AYPCA) categories to use when ranking the defenses. For example, four teams since 1960 have pulled off the rare 1-1-1-1. Here's a look at them.

2002 Bucs: They made Super Bowl champions out of Brad Johnson and John Gruden. This defense also scored five TDs in the regular season and four more in the playoffs. 196 points allowed, 1.1 OTA per game, 2.34 ANYAA and 4.18 AYPCA; NFL averages that season were 352 points allowed, 2.3 OTA, 5.47 ANYAA and 4.89 AYPCA.

1986 Bears: Not the Super Bowl version, mind you; if there's a blemish on that defense it was that they ranked 6th in yards per carry allowed and 5th in AYPCA. The '86 version allowed 187 points, 1.0 OTA/G, 2.63 ANYAA and 3.61 AYPCA. The league averages were 333, 2.2, 5.02 and 4.56.

1969 Vikings: Post-season failures by the team enabled this dynastic defense to fade into oblivion. The '69 Vikings went 1-1-1-1 in our four categories; the '70 version went 1-1-1-3 and the '71 team went 1-1-2-2. The Vikings had a three year stretch that was never matched. If you add their ranking in each season, you get a total of sixteen. The next best stretches are the '84-'86 Bears (24), the '86-'88 Bears (27), the '85-'87 Bears (27), '74-'76 Rams (30), '62-'64 Packers (33), '72-'74 Steelers (33). The closest modern teams are the '95-'97 49ers (38), the '99-'01 Ravens (43), '02-'04 Bucs (50) and '04-'06 Ravens (51).

The '69 Vikings allowed 133 points, 0.9 OTA/G, 0.96 ANYAA and 3.47 AYPCA. NFL averages were 301, 2.3, 4.70 and 4.62.

1963 Bears: George Halas' last championship squad. Signature game of the year? In Los Angeles, Mike Ditka caught four scores while the defense forced eight turnovers. 144 points allowed, 1.2 OTA/G, 0.81 ANYAA and 3.84 AYPCA. NFL (and AFL) averages were 322, 2.6, 4.93 and 4.77.

Three other teams have ranked last in each of the four categories. Here's how each of the teams graded along with the NFL averages on the right:

		PA	OTA/G	ANYAA	AYPCA		PA	OTA/G	ANYAA	AYPCA
atl	1992	414	2.8	6.89	5.81	||	296	1.9	4.81	4.52
tam	1986	473	3.4	7.08	5.86	||	323	2.2	4.87	4.47
nwe	1972	446	3.6	7.52	5.94	||	277	2.0	4.15	4.68

A couple of things are noteworthy about the bottom two teams on that list. The Bucs defense was so bad that it caused Steve Young to go 2-12 as a starter. That '72 Patriots defense? Despite having a bad week just about every game they played, they allowed 501 yards in one game against the perfect Dolphins and lost 52-0 in their other game; that series was most definitely a sweep.

As for the Steelers? By the slimmest of margins, Pittsburgh is currently pulling off the 1-1-1-1, but the Ravens are nipping at their heels in the OTA/G category, the Titans are right there in points allowed and the Vikings are close in AYPCA. They're not the only ones making history, though: the Detroit Lions are 32-32-32-32 to date, and things at the back aren't even that close.

This entry was posted on Wednesday, December 17th, 2008 at 9:17 am and is filed under History, Statgeekery. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.