Defensive Efficiency: Yards Per Point Allowed and Yards Per Drive
This is the second post in my series discussing Defensive Efficiency Scores (D-SCORE). Here’s the first part if you missed it:
Having looked at Points Per Drive (PPD) and Touchdown %, we turn to a somewhat deceiving – and overused – stat: yards allowed. Too often, defenses are ranked and judged by the yards they allow. It’s a very poor and misleading way to evaluate defenses. After all, yards don’t mean much if they don’t lead to points.
The first way of thinking about yards, is by comparing them to points allowed. Taking a team’s Yards Per Point Allowed (YPP) is one measure of how efficient a defense is at preventing points.
This graph shows that defenses were stingiest in the late 1970’s and early 1990’s, but since 1994 things have settled into a fairly consistent 15.0-15.5 range. The drop in average YPP was a combination of rule changes (moving the kickoff back to the 30 yard line from 1994-2010), the increase of pass heavy/spread offenses and arguably better league-wide offensive talent (both players and coaches). Here are the best and worst YPP defenses since 1970:
As with offensive efficiency, Yards Per Point is a big piece of the puzzle. It’s rare for a team to rate well in PPG+ and poorly in YPP+ (about 10% of teams since 1970 meet that criteria). This is because teams who rank poorly in YPP tend to give up a lot of TD’s. As I mentioned in the points per game vs. touchdown % discussion, TD% is the biggest variable in inflating points allowed (a bit obvious since a TD is worth more than double the points of a FG). So teams who rate well in YPP tend to be those who can clamp down and force FG attempts, turnovers or punts – even if they give up a bunch of yards. It would be almost impossible for a team to score well in YPP and give up a lot of TD’s.
Another way of looking at yards is Yards Per Drive (YPD), which in itself doesn’t tell us much about the quality (or efficiency) of a defense. In an ideal world, a defense would be excellent at preventing both yards and points. That doesn’t mean that defenses which give up lots of YPD are necessarily bad. However, of the 100 worst defenses since 1970 in terms of YPD+ (yards per drive allowed relative to league average), only 11 have been above average in preventing points (measured by PPG). Conversely, of the 100 best defenses in YPD+ only 4 were below average in allowing PPG. That said, once you get into the middle of the distribution, the correlation between YPD and PPG breaks down.
Truly great defenses are good at preventing both yards and points.
Truly terrible defenses give up tons of both yards and points.
There are plenty of “pretty good” defenses which give up more yards than you would expect based on their PPG allowed.
Because the vast majority of defenses fall in the middle of the distribution, using YPD as a measure of defensive quality is fairly useless.
Take a look:
What’s notable here is that between 90-110 PPG+ (i.e. league average ± 10%) we see a big range of YPD+ (roughly league average ± 30). The white trend line bears this out, giving us an R2 of 0.4. In other words, about 40% of the variance we see in Points Per Game allowed is related to Yards Per Drive Allowed. The other 60% comes from different (non yardage based) variables.
More proof that YPD allowed (or YPG allowed) isn’t a great measure of defensive quality: check out this comparison between defensive DPPG+ to YPD+:
The “Diff” column all the way to the right shows the difference between DPPG+ and YPD+. Teams with big (positive) differences between these two variables tend to be “bend but don’t break” defenses – teams who give up lots of yards but not lots of points. This is especially true for any team with a YPD+ of less than 100. There are a few teams (the 1971 Vikings, 1975 Rams, 2000 Ravens etc) who were elite in both categories and probably shouldn’t be lumped in with the “bend but don’t break” teams. The #1 team on this list, the 1973 NFC Champion Minnesota Vikings, allowed 39% fewer points than the average team that year but allowed 9% more Yards Per Drive. Were we solely looking at yards, we’d probably consider the ’73 Vikings to be a pretty bad defense – but that’s not the case at all. In fact, they are 17th (out of 1189) since 1970 in PPG allowed.
Here’s the other end of the spectrum:
There are eleven teams here which were better than league average in YPD but well below average in PPG. The 2000 Chargers really stand out. They were 6th best in the league in YPD+, allowing 12% fewer yards per drive than the average team. However, they were 28th (out of 31) in PPG allowed – giving up nearly 25 points a game (28% worse than league average).
In summary, YPP is a valuable measure of defensive quality. YPD (or YPG) is not particularly useful, at least not without additional context. Unfortunately, you never hear NFL analysts using the YPP (or Points Per Drive) statistic. History is littered with teams who gave up lots of yards but were pretty good (if not great) at preventing points. So next time you hear someone on TV going on and on about how bad a defense is because of the yards it gives up, remember: yards don’t mean anything if they don’t result in points.
The next part of this series will focus on turnovers – something a lot of these “bend but don’t break” teams rely heavily upon.