Predicting the Bears Defense

Last Sunday's loss to the Bills stung. More than anything, I was bummed to see so little of Jared Allen, Lance Briggs, and Charles Tillman -- the veterans who I expected would at least perform up to recent standards. (Of course, of those three, Tillman not having any big plays could very well be just him having a solid game and pushing plays towards the other DBs.)

As a whole, though, I am worried like most Bears fans that the defense is poised once again to lead the league yards allowed per play and likewise finish as or near the bottom in points allowed per game. In 2013, only the Vikings were worse than the Bears in preventing scores, and they were worse by only 2 points. Ouch.

But here's a fun fact: Almost all of point scoring can be captured by two basic defensive stats.
  1. Y/P: Yards per play, or the total yards allowed by the defense divided by the number of defensive plays.
  2. Start: The average starting position for the defense -- i.e. where the defense takes over after the most recent Jay Cutler interception.
Given 2013's numbers, these three statistics explain almost all of the variations in points allowed per game (82% or 0.82 R-squared). And if we run a linear regression on these items, we get a formula like this:

Points/Game = (6.72 * Y/P) + (1.12 * Start) - 44.2
Using this really basic formula, we can make primitive predictions about the Bears defense (Note: There is no shortage of complex and more reliable predictions on Advanced Football Analytics and Football Outsiders; go there for those aims).

So far in 2014, the Bears have allowed 6.4 yards per play and have had to start on the 32.9 yard line. What's fun about these stats is that they represent components controlled by the defense (Y/P) and the offense and special teams (Start).

Given they continue their previous performance, the Bears defense will allow something between 30 and 42 points per game in a typical Sunday. That's not good.

But let's say the offense is what we thought it is, and the average starting position moves to a league average 28-yard line. The defense then improves to a 25 to 35 range -- much better, especially if the offense is scoring more than 20 points. Looking at it from the other direction: What if the field position issue doesn't resolve, but the defense manages to reduces the yard-per-play issue? With a league average 5.4 Y/P, the Bears would give up somewhere between 24 and 34 points per game -- almost the same impact as a league average starting position.

This leads me to suspect this is not just a defense problem. The Bears offense must sustain drives and give the Bears defense a better chance. Both sides (and special teams too) appear to blame so far.

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