Defensive Efficiency: Points Per Drive And Touchdown %
An obvious point: a defense’s primary goal is to prevent points. Too often, NFL “experts” and analysts will spew something like this “Team X has the 32nd ranked defense, they’re just awful! You can’t win a Super Bowl with a defense that bad!”. You’ve probably heard it a lot this year in reference to the Patriots and, perhaps, the Packers. Both teams are giving up almost unheard of amounts of yardage. Yet, I wouldn’t call either defense the “worst” or even “extremely bad”. Why? Well, games aren’t scored in yards but in points. Now, it’s true that defenses which give up tons of points tend to give up tons of yards. After all, it’s tough to score lots of points without long, touchdown producing, drives. However, that doesn’t mean that teams which give up piles of yards give up lots of yards.
An easy, and recent, example of this are the Super Bowl winning 2009 New Orleans Saints. They were 25th (out of 32) in terms of total yards allowed – about 6% worse than league average – but were 5% better than league average in points allowed per drive. They’re not alone in being significantly better in points allowed than in yards. In fact, that 11% swing is only 194th best since the 1970 AFL/NFL merger. Take a look at the 20 biggest differences between Defensive Points Per Drive Allowed (DPPD) and Yards Per Game (YPG):
The 1984 Denver Broncos (who went 13-3) are a great example of a team that was “bend but not break”. They gave up 7% more yards than league average, but allowed 30% fewer points per drive than average. Bill Belichick coached teams appear twice in the Top 20 (1994 Browns, 2003 Patriots). The 1993 Super Bowl losing Bills gave up a whopping 347 yards per game (27th ranked out of 28 teams) but only allowed a stingy 1.41 points per drive (6th best in the league).
As I touched on in my discussion of offensive efficiency, the single biggest factor (unsurprisingly) in the amount of points per drive scored/allowed is touchdown percentage. It’s a pretty solid 10:1 ratio. In other words, if a team allows a touchdown on 15% of their drives, we expect them to give up roughly 1.5 points per drive. That ratio breaks down a little bit towards the extremes of the distribution, but as a general rule of thumb it holds pretty well. Here it is in graphical form:
Points Per Drive has gone up a little in the last 40 years (about 0.4 PPD):
And T0uchdown % over that same time period:
Lastly, here are the best and worst TD% defenses of all time:
In the “Best” group, we see the 2000 Ravens ahead of the 2000 Titans. We also see a better mix of time periods represented than we did in the Adjusted PPG list from yesterday. The ability to put together a stingy defense hasn’t really changed much over time, even if league-wide scoring is up a bit. A good example of how things can change over time is the 2002 Super Bowl Champion Tampa Bay Buccaneers who allowed 52% fewer TD’s than average. That’s 1% better than the 1977 Falcons, despite the 2002 Tampa Bay team giving up a TD on 2.29% more drives. Taking that a step further, that’s about 30% more frequently. Sounds a bit crazy, but it demonstrates the importance of taking teams’ performance relative to the league average for that given year. It’s the only way to properly account for league wide rules changes and trends.
The “Worst” group is a collection of appallingly bad defenses, none worse than the 1981 Colts who gave up a TD on 38% of their opponents offensive possessions. That is a staggering 91% worse than the average team allowed in 1981.
Lastly, using TD% as opposed to overall scoring % may seem a bit odd to some. I mostly ignore FG% for a few reasons:
1. Field goals only represent about 19% of total points scored (about 4-4.5 points per game per team).
2. Oftentimes, holding an opposing offense to a field goal can be a “success”. Imagine a scenario where a team throws an INT at their own 15 yard line. If their defense prevents their opponent from turning that very short field into a TD, they’ve done their job. Obviously, that’s not always the case though (e.g. a team driving down in the last minute of the game for a game winning field goal). Unfortunately, it’s difficult – if not nearly impossible- to filter out “good” field goals allowed and “bad” field goals allowed. It’s best to assume that these scenarios will offset somewhat over the course of a big enough sample size.
3. A field goal is not a guaranteed score. The defense still has a chance to block the field goal, or the kicker could miss. A touchdown is always a guaranteed score, so it should get significantly more weight.
The takeaway points here are that, as with the offensive efficiency rankings, Points Per Drive are the biggest thing to look at. TD% is one of the biggest determining factors of PPD but it’s not the only one. Teams which give up lots of TD’s and PPD tend to give up lots of yards but not all teams which give up lots of yards give up TD’s and PPD. Some defenses tend to give up yards but clamp down when their opponent gets near the red zone. Other defenses allow long drives but are adept at generating turnovers (and thus, limit the points they allow). Yards Per Point and Turnover % will be the subject of the next discussion.