Power rankings are always debatable. I don't buy into the gut feel methods most places use to determine their rankings, so I developed a formula a few years back that attempts to take at least some of the subjectivity out of the discussion. My approach is simple, I measure offensive and defensive efficiency based on the Yards Per Carry (YPC) and Yards Per Attempt (YPA), as well as points scored and points allowed. The formula to calculate "Team Strength" is as follows:
(YPC (offense) + YPA (offense) + Avg Pts/Game Scored) - (YPC (defense) + YPA (defense)+ Avg Pts/Game Allowed)
The formula has proven to be a pretty accurate predictor of success. Even in the first week of the 2008 season, 5 of the top 10 ranked teams were playoff bound. As with any statistic, it becomes more meaningful as the sample size grows. Usually, these become most meaningful after Week 3. In 2007, 9 of the top 10 ranked teams were playoff teams, with the lowest ranked playoff team coming in at #15. In 2008, 8 the top 10 were playoff teams, with Arizona being the lowest ranked playoff team at #19. I'm not sure any formula could have predicted their run.
If you'd like to see how teams rankings changed from 2008 to 2009, you can read more here.
UPDATED: MNF game stats now included. San Diego rises to #3 overall. They are a classic case of the record not reflecting the true strength of the team. They have not left the Top 10 of these rankings all season, and have often been in the Top 5. They will win the division, and be a tough out in the playoffs. New Orleans is sneaking up the rankings.
Scatter chart of the rankings. This view helps to give you a view of how teams are grouped together. You will generally see tiers of strength develop as the season wears on.