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On Monday, we introduced our special Football Outsiders "30 for 30," a look at the 30 best offenses of the past 30 years now that we have our advanced play-by-play metrics going all the way back to 1987. In this installment, we'll turn things around and look at the 30 highest-rated defenses. Football Outsiders' DVOA (defense-adjusted value over average) metric accounts for every aspect of defense: how well teams prevent yardage, how often they get turnovers, and how well they keep points off the board. DVOA measures success on each play, defined using down and distance, then compares it to an NFL average baseline adjusted for situation and opponent. You can read more of the details here -- and yes, defensive DVOA adjusts for opposing offenses, not defenses, but we still call it DVOA. The metric is built to balance a measurement of how well a team has played in the past with a forecast of how well a team will play in the future. Just like our list of top offenses, a couple of franchises really stand out as consistently great. Where the list of top offenses was dominated by San Francisco and New England, this list of top defenses is dominated by the AFC North rivals Baltimore and Pittsburgh. They combine to represent nine of the top 30 defenses since 1987. You can find DVOA stats for all 30 seasons on the stats pages at Football Outsiders, but as we noted Monday, these stats will be a little different from those regular-season ratings. For the first time we've also added in postseason performance, boosting teams that went on strong championship runs and lowering teams that dominated the regular season only to trip over their own feet in the playoffs. Ratings each year are normalized, accounting for changes in the NFL's offensive environment over the past 30 years. It was certainly easier to keep quarterbacks contained in 1996 than it was in 2016. Note: A negative defensive DVOA is good and 0 percent represents the league average, so a defense with a minus-30 percent DVOA rated 30 percent better than an average unit. Also, stats other than total defensive DVOA represent the regular season only unless noted. Special thanks to Jeremy Snyder, who did most of the transcription work on the late 1980s and early '90s.
 30. 2004 Pittsburgh Steelers-18.6 percent Pittsburgh has ranked No. 1 in defensive DVOA five times, more than any other franchise during this 30-year time frame. This was not one of those years, as there will be two other teams from 2004 listed below, but this Steelers defense rates better than the No. 1 defenses they had in both 1993 and 1994.  29. 2008 Tennessee Titans-18.9 percent This is how a team goes 13-3 with a 36-year-old Kerry Collins at quarterback. The Titans would rank even higher if they had not rested a number of starters in a meaningless 23-0 loss to Indianapolis in Week 17.  28. 2016 Denver Broncos-19.0 percent Last year's Broncos were even better against the pass than you probably think. Only two defenses in 2016 faced a tougher schedule of opposing offenses. Nine of Denver's games came against the top dozen teams in pass offensive DVOA, although the Broncos also get a bit of an artificial boost because one of those games was against Oakland's backup and third-string quarterbacks. The 2016 Broncos rank as the No. 12 pass defense of the past 30 years, but they get dragged down our rankings in overall defense because they were below average against the run. 
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