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Pro Football Focus uses a complex system to analyze every NFL player involved in every snap in every game and assigns a grade based on how he performed on that play, with each position having its own set of grading guidelines. Plus/minus scores are given in 0.5 increments and a small normalization factor is applied to make 0.0 the average grade for a game or season. To learn more, visit www.profootballfocus.com

As the Detroit Lions fortunes have tumbled this season, so have those of quarterback Matthew Stafford. Last year, Stafford's -5.0 PFF passing grade ranked 21st in the NFL. Fast forward to 2015 and Stafford's -16.8 grade has him ranked last out of 36 quarterbacks.

You won't see that drop-off using the traditional measure of quarterback performance, as Stafford's 2015 NFL passer rating is actually on par with last season's (84.1 vs. 85.6). So what's the deal?

It starts with the type of throws. The PFF grading system awards higher grades for throws of higher difficulty, and Stafford isn't attempting as many this season. He has been targeting more slants and crossing routes than he did in 2014, and fewer go routes and post routes. As a result, his average depth of target has dropped significantly from 8.2 yards to 7.2. Only 8% of his attempts are deep throws (targets 20-plus yards downfield), which is down from 10.5% last season.

The quality of the throw comes next. On the edges of PFF's grading system are "Big Time Throws" and "Turnover-Worthy Plays." BTTs are accurate, well-timed downfield passes. TWPs are bad decisions or inaccurate passes that are intercepted, or could have been intercepted, and fumbles in the pocket because of poor ball security. Last year, Stafford's BTTs outweighed his TWPs (27 vs. 17). This year, things are flipped (12 BTTs vs. 20 TWPs). In contrast, Packers quarterback Aaron Rodgers has 21 BTTs vs. only 7 TWPs.