Introducing ESPN's new QB rating system

Tonight on a SportsCenter Special (8 p.m.), ESPN will unveil a new statistic that will rate NFL quarterbacks: The Total Quarterback Rating.

We've long known the traditional quarterback rating was a flawed statistic, so a bunch of numbers gurus bunkered down in Bristol to figure out a better one. Here are a few of the details of the new rating (we'll try to keep it in layman's terms):

* QBR is scaled from 0 to 100, with 50 representing league-average performance. For a single game, a rating in the 90s is terrific; last year, Michael Vick's six-TD Monday night symphony against the Redskins topped the charts at 99.8. For a season, any QBR above 65 is Pro Bowl-caliber, and Tom Brady led all starters with a 76.0 QBR in 2010.

* Not all pass completions are created equal, and this new system incorporates game situations and level of difficulty for each throw.

* QBR starts with this insight: Any possession in a football game has an expected value -- the average number of points the team with possession can expect to score, based on all the historical outcomes for teams facing the same down, distance, field position and time remaining. And that means we can evaluate any play by how much it increases or decreases a team's expected point total.

* QBR allocates the points added by every play in an NFL season to each of the players involved, every play. On completed passes, for example, it splits credit among QBs, receivers and blockers, depending on factors such as whether the quarterback was under duress, where he threw the ball, how far it traveled and how many yards the receiver gained after the catch. QBR splits the blame for sacks on quarterbacks and offensive linemen and attributes QB fumbles to QBs. Further, QBR weights every play by its clutch value -- its contribution to a team's chances of winning, given the score of a game, not just to scoring points.

We know it's a little confusing on the surface. Here's a link with more details on exactly how QBR is calculated.

And see below for the QBR for each quarterback last season. As we mentioned above, Brady tops the list.

And here are the top-rated individual seasons from 2008-2010: