Start Baltimore Ravens, John Skelton

This column is obviously predicated on having reliable matchup information available. At Football Outsiders, we use our advanced metrics to get a better idea of what's really going on with a defense and determine the causes of a particular performance. As one example, while a 360-yard passing day might suggest that the defense played poorly, our analysis may very well chalk it up to the effects of an early lead and realize that the yardage isn't likely to recur when the team plays a closer game the week later.

However, there are times when a team's performance changes drastically during the season and our cumulative numbers for the season disguise a stunning trend. Take last year's Denver Broncos, for example. They finished the year a respectable 18th against the run. That belied an incredible split, though: The Broncos were the second-best team against the run in the first half of the 2009 season, but over the final eight weeks of the year, they were the second-worst team in football against opposing rushers. That would have been pretty useful to know for owners who were thinking about starting Michael Bush in Week 15 or Jamaal Charles in Week 17 last year.

With the second half of the season halfway through, we've identified eight teams who have either seen their run defense or pass defense drastically change over the past four games. While some of the change could be due to a small sample, we'll also try and identify if there are outside factors -- like a change in style or the effects of injury -- suggesting that the change is real.

For all of Barnwell's advice here, you need to be an Insider.

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Bill Barnwell

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Bill Barnwell (@billbarnwell) is a staff writer for Grantland.