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Quarterback rating is a statistic every football fan knows but few really get. Why is it scaled so that 158.3 is a perfect score? Why does it only include passing, and not other aspects of quarterback play like avoiding sacks and running? Why does it penalize interceptions but not fumbles? Why does it treat a six-yard completion on second-and-5 the same as a six-yard completion on third-and-11?

The NFL adopted passer rating as an official statistic in 1973, and the quest to improve passer rating began almost immediately. Now ESPN is poised to take the biggest step yet toward an improved rating that will be both a better assessment of how good a quarterback is and an easier stat for average NFL fans to understand.

The new stat, which ESPN is calling the Total Quarterback Rating (Total QBR), will be unveiled during a TV special on Friday night. Developing Total QBR was a joint venture between some of the statheads at ESPN and NFL analysts like Trent Dilfer, Jon Gruden and Ron Jaworski. The bottom line, as one of ESPN’s statisticians put it in a press release, is, “If you want one stat that measures the totality of a quarterback’s performance, it’s QBR.”

Unlike passer rating, Total QBR considers everything a quarterback does except handoffs. Quarterback runs, sacks, fumbles and penalties, all of which are ignored by passer rating, are included in Total QBR.

Total QBR also includes the situation of the play, such as down and distance, field position and the time left in the game. So a quarterback who runs for two yards on third-and-1 will be rewarded in the system, while a quarterback who runs for two yards on third-and-10 will be penalized. And a quarterback who racks up a bunch of passing yards while his team is way behind in the fourth quarter, playing against a prevent defense, won’t be treated the same as a quarterback who racks up a bunch of passing yards in the process of helping his team build a lead in the first half.

One of the aspects of Total QBR that could be both a strength and a drawback is that it considers data that the average fan doesn’t have access to, like how far a pass travels in the air, and whether the quarterback was under pressure when he threw it. That could be a great benefit of Total QBR because it incorporates detailed information that only comes from film study, not from the box score. The drawback, however, is that it means fans can’t see for themselves exactly where Total QBR comes from — fans just have to trust that the distance the ball traveled was correctly measured, and how much pressure the quarterback felt on the play was correctly assessed.

Despite all those extra elements that go into Total QBR, the stat is designed to be easier to understand than traditional passer rating. Toward that end, Total QBR is based on a scale where 100 is perfect and 50 is average. No more perfect scores of 158.3.

If you’re curious how Total QBR works in practice, Mike Sando of ESPN.com writes that the top quarterbacks of 2010, according to Total QBR, were Tom Brady, Peyton Manning, Matt Ryan, Michael Vick, Aaron Rodgers and Drew Brees. The worst quarterbacks, according to Total QBR, were Derek Anderson, Brett Favre and Jimmy Clausen.

That seems roughly correct, but then again you didn’t need an advanced stat to tell you that. And right now we don’t know all the details of exactly how this stat is calculated.

If ESPN is committed to this stat and is able to clearly and concisely explain it on the Worldwide Leader’s NFL broadcasts, then fans will quickly become familiar with it and it will soon become a staple of how we talk about quarterbacks. On the other hand, if the stat comes across as too convoluted — or if it doesn’t really seem like much of an improvement on the current passer rating — then this will all feel like a rather pointless exercise. We’ll be interested to see how it’s presented on Friday night. And we’ll be interested to hear what PFT Planet thinks.