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Football Outsiders -- 2013 Quarterbacks: True Sack Rate
Arguably, the worst play a quarterback can make is throwing an interception. However, there are high amounts of luck and randomness involved with the rate at which interceptions occur. Sometimes the pass was tipped by the intended receiver. Sometimes the quarterback gets hit as he's throwing. Then you have the dozens of awful passes each season that should have been intercepted, but were dropped by defenders (reminding us why they play defense).
There's another type of play with negative consequences that is a much better predictor of future quarterback performance. The sack is a more common event that carries far less variation than the myriad of possibilities that come after a pass leaves the quarterback's hand. On these plays, the whole problem is the ball never did come out, and if it did, then we're looking at a fumble too.
Yet in all my years of following the NFL, it always seems like the sacks and fumbles are ignored in favor of the interception when it comes to analysis of a quarterback's mistakes. The sack is almost like a litmus test for where a fan is in their understanding of the game. Usually you start out thinking it's all about the offensive line and protection, but the sack is actually very dependent on the quarterback...