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Football Outsiders on the Pass Defense

I asked 5 questions to Bill Barnwell at Football Outsiders based on the chapter on the Green Bay Packers in their Football Outsiders Almanac 2009, which is one of the many great publications they have available for the upcoming season. I'm going to split it up into five separate posts over the course of this morning.

My first question: There was a remarkable amount of luck with the defense that turned interceptions into TDs. But does that mean that the overall number of interceptions was luck, or is it likely the defense will have another season with 20 or more INTs?

That's a great question, and something worth noting and clarifying. Intercepting passes is, to an extent, a skill, but team interception chances are dictated more by the number of passes that are thrown at them. 

We know that the Packers had 22 interceptions a year ago. You have to adjust that for the number of passes thrown at them; after all, 22 interceptions means a lot more when the opposition throws 400 passes as opposed to 600. Green Bay intercepted 4.2 percent of opposing passes, the fourth-highest percentage in the league (behind Baltimore, Tampa Bay, and strangely, league-leading Cleveland).

There have been 51 other teams that averaged interceptions at a level of between 4.0 percent and 4.2 percent of opposing passes since 1989. In the season following that interception rate, those teams averaged an interception on 3.4 percent of opposing passes; the league average rate is 3.3 percent. So, in other words, those teams aren't really anything special in subsequent years.

If the Packers had picked off 3.3 percent of opposing passes a year ago, that would have resulted in just about 17 picks. That's probably a more reasonable expectation for their 2009.