It would not come as a surprise to most fans of the Dallas Cowboys to discover that over the last three seasons, the Cowboys have consistently been one of the worst pass defenses in the NFL as measured by defensive passer rating. In fact, only five other teams have been as consistently below average as Dallas:

The NFL average defensive passer rating over the last three seasons has been remarkably consistent. Below is the average from 2009 through the 2011 season. As previously described, none of the teams highlighted above have finished with a defensive passer rating better than the average over the last three seasons.

The afore mentioned teams are remarkably similar in several statistical categories. Those teams compare favorably to each other. Those teams have also combined for two playoff wins since 2009 (Cowboys over Eagles, and Vikings over Cowboys in 2009).

Utilizing defensive passer rating as the differentiating factor between pass defenses in the NFL, Dallas is ranked in the bottom 20% of the league over the last 3 seasons. Some of the teams that rank among the leaders include the New York Jets, the Baltimore Ravens, and the Green Bay Packers. Those are the only three teams that have posted a defensive passer rating better than the average each of the last three seasons. Those three teams have the following defensive passer rating:

To expect the Cowboys to drop their defensive passer rating three-year average of 86.2 to 66.8 (a 19.4 decrease) is asking for Dallas to accomplish a feat that has only been completed once over the last three seasons: by Seattle (a 18.6 decrease from 2009 to 2011).

Other teams, such as Houston, had a defensive passer rating in 2009 of 83.2, plummeted in 2010 to 100.5, and then improved to 69.0 in 2011. The question becomes, did Wade improve a defense that had a rating of 100.5 to a defense that posted an 69.0 defensive passer rating? Or did Wade take a good pass defense that had a bad 2010, and turn it into an above average unit?

The Seahawks, however, provide a template for the Cowboys when combined with the other 15 teams that have finished just one season under the average defensive passer rating since 2009. After all, this is what Dallas hopes to accomplish this season with the upgrades made to the secondary.

Half of the teams in the league have finished only one season better than the NFL average for that season in defensive passer rating since 2009. In the season that those teams finished below the NFL average, their collective defensive passer rating averaged 5.43 points below the NFL average.

The anticipated average NFL defensive passer rating in 2012 should be close to 82.0. If Dallas becomes the 17th team to have one season below the average defensive passer rating since 2009, the Cowboys' expected rating in 2012 should approximate 76.56. That would constitute a modest drop of 6.94 rating points.

Unlike the previous grouping to which Dallas now belongs (the original six teams mentioned in this post that have consistently finished with a passer rating on defense worse than the NFL average), the sixteen teams that have finished only one season better than the NFL average for that season in defensive passer rating since 2009 includes 10 teams that have participated in the playoffs since 2009. There is a significant difference between the teams that never have finished with a better than average defensive passer rating (since 2009), and the teams that have posted at least one better than average season playing pass defense.

An improvement of about 7.0 points in defensive passer rating could be the difference between watching the Cowboys in mid-January, and talking about the 2013 NFL draft. Perhaps the additions of Brandon Carr and Morris Claiborne will make the difference needed.