[Editor's note: frontpaged for obvious reasons. A scheduling mix-up with Brian caused this to get buried earlier, so I'm bringing it back near the top. [How much] Will Michigan rue the loss of Brandon Graham? - Tim]
[Note: I have 2006 fully loaded into the database now and will be included in all future multi-year studies along with 2007-2009.]
We can all agree that sacks and interceptions are good things for the defense and bad things for the offense. But how does a viable pass rush or a ball-hawking secondary affect the performance of the opposing offense on plays where there isn’t a sack or a pick. Likewise, what is the correlation between an offensive line that gives up sacks regularly or a mistake prone quarterback?
Sacks and interceptions have very similar direct impacts on games. From 2006-2009 in games between two D1 teams in competitive game situations (the “universe” for this and most of my analysis) the average defensive unit produced 2.3 ppg worth of sacks and 2.0 ppg worth of interceptions. Sacks have a slightly higher direct value than interceptions (interceptions returns and fumble returns on sacks are not included) but does either of these correlate to a better defensive performance overall.
Chart time? Let’s make it a double.
Not entirely surprisingly, the better a defense is at producing sacks and interceptions, the better it is on downs where neither occur.
For every point per game that a defense generates due to sacks, the overall pass rush generates 1.2 ppg of additional value. Interceptions are also powerful, but not as much so. Each ppg of value a defense generates through interceptions is worth 0.9 ppg of additional value.
This analysis serves to confirm what most football fans already know. Teams that can create interceptions and sacks are good going to be better defensive teams. Whether a strong pass rush/secondary creates pressure on other downs or if strong pass rushes and secondaries are a common occurrence on great defenses is irrelevant. As most of you probably know, defenses that are good at these two things are also good on other downs. So why is this interesting…
The story becomes very different when you look at offenses. The conventional wisdom that was supported for defenses is largely blown up on the offensive side of the ball. Sacks and interceptions may be indicators of great defenses, but they are not symptoms of bad offenses.
The slope of these two charts are about 20% of the gradient of the corresponding defensive charts and virtually flat. On offense, the amount of sacks and interceptions are largely independent of performance. There is obviously the immediate negative effect of the play, but giving up sacks or throwing interceptions show virtually no correlation to success or failure on other downs.
What it means?
For one of side of the ball it merely quantifies conventional wisdom. Good pass defenses get interceptions and sack the quarterback and teams that get interceptions and sack the quarterback are often good pass defenses, even on other plays. The value they create is roughly equal to value created by the big plays.
On offense, it’s a very different story. Interceptions and sacks will always be bad plays for an offense, but their rate of incidence is not strongly correlated to performance on other downs. In fact, if given the choice between a quarterback who threw a lot of picks the prior year but was generally successful otherwise and a quarterback who was very safe but not all that productive, my guess is you will be better of going for the quarterback with the picks.
Special thanks goes to Ty and The Lions in Winter who has been working on a similar line of reasoning for the Lions revamped defensive line.
Potential Future Diaries
Just some ideas I am kicking around or have half started. Let me know what you think about these or any other things you would like to see.
A follow-up piece on fourth downs digging deeper into how the decision making changes based on the relative strengths of the offense and the opponents defense
A broader look at “luck”, looking back over the last four years.
When are extra yards not worth it. The secret dead zones of football.
Probably not for several months, but a big season preview is in the works.
Something Carr vs. Rodriguez, now that I have 2006-2007 seasons of data I have two years to compare the two more directly.
How the best players of the last four years (TEBOW!!!) progressed over the years. Maybe a companion piece on Michigan defenders.
Any other suggestions? An article a week means I need all the ideas I can get, I’m not afraid to beg!