How to Tell If Your Team's Defense Will Improve This Year

The most commonly used way to measure the quality of a defense is actually a pretty good and valid system. That measurement, which determines defensive rankings, is the amount of yards that a defense gives up. It's simple, and it's to the point. After all, what else is a defense asked to do than stop the other team from gaining yards? Points given up is a slight bit trickier. After all, some defenses often start with worse field position than others and thus, could give up few yards but still yield some points.

For the most part, however, a good defense will give up fewer points than will a bad defense. The data supports this assertion. There is a staggeringly high correlation of 0.768 between the amount of points and the amount of yards given up by a defense.

However, there are many instances in which, over the course of a season, good defenses will give up slightly more points than they should, and bad defense will give up slightly less. One key determinant of how many "extra" points a good defense gives up or how many points a bad defense "saves" is the amount of takeaways that a defense has. A regression analysis was performed using the variables of yards allowed and takeaways as independent variables and points allowed as dependent variables. In other words, I ran an equation to see if both yards allowed and takeaways would each be significant predictors of the amount of points that a defense allowed. The results indicated that both factors are indeed significant predictors of the amount of points that a defense allows.

So, what determines how many takeaways a defense has during a season? Would you believe luck? Well, that appears to be the answer. There is nor correlation whatsoever from year to year over how many takeaways a team will have. In other words, there is no known way to predict even approximately how many takeaways a team will have the next year. For instance, Baltimore led the league in takeaways in 2006 with 37. In 2007, they finished 27th with 23. Meanwhile, San Diego had only 27 takeaways in 2006 but led the NFL in 2007 with a whopping 48 takeaways. The complete lack of consistency implies that chance is the primary determinant of how many takeaways a defense will have.

The law of averages dictates that eventually, everybody's luck evens out. Therefore, if chance is the primary factor in takeaways, a team with an otherwise good defense that got unlucky with few takeaways should experience an improvement in points allowed the next year. On the other hand, some teams that gave up a lot of yards but got a lot of lucky takeaways should see its yards allowed go up this year.

Of course, offseason personnel and coaching changes can naturally change a defense, as well. However, a team bringing back basically the same defensive squad should fall prey to these laws. Below are three teams whose defensive performances were better than their points allowed because of bad luck in takeaways. All other factors being equal, these teams may expect to allow fewer points this year.

Baltimore Ravens- 302 yards allowed/game; 23 takeaways; 24.0 ppg allowed
New York Giants- 305 yards allowed/game; 25 takeaways; 21.9 ppg allowed
Kansas City Chiefs- 319 yards allowed/game; 22 takeaways; 20.9 ppg allowed

The three teams below had poor defensive years but allowed fewer points than expected because of a large number of takeaways. Expect these defenses to give up more points this year:

Buffalo Bills- 363 yard allowed/game; 30 takeaways; 22.1 ppg allowed
Minnesota Vikings- 335 yard allowed/game; 31 takeaways; 19.4 ppg allowed
Chcago Bears- 355 yard allowed/game; 33 takeaways; 21.8 ppg allowed