Quarterback play is undeniably the most important variable that drives team success. If a team has a tremendous quarterback, it almost has to consciously try to fail.
That being said, quarterback play is incredibly difficult to isolate using just data. For a QB to complete a pass, his coach needs to draw up a good play, the defense needs to be put into a bind, his offensive line needs to hold up, and his wide receivers need to get open. If most of those conditions aren’t met, not even Kansas City Chiefs QB Patrick Mahomes could succeed on a snap-to-snap basis.
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Quarterbacks play some role in determining all of these. Defensive coordinators base their calls partially on the passer they’re facing. A great QB can manipulate the defense to generate separation, and signal callers play a significant role in the pressure they see. That being said, QBs are still partially dependent on how strong their surroundings are. NFL passers tend to have surroundings that are close enough to equal, so while surroundings shouldn’t be neglected, it is much more justifiable.
This does not hold true in college football. As an example, the UTEP Miners had two players drafted in the 2010s (Aaron Jones and Will Hernandez) while the Alabama Crimson Tide’s 2017 recruiting class had three future first-round WRs (Jerry Jeudy, DeVonta Smith, and Henry Ruggs III).
Since massive talent disparities are inherent in college football, we cannot claim surroundings are functionally equal. Therefore, we cannot easily make apples-to-apples comparisons of QBs with metrics such as PFF grades.
This has massive implications for the NFL draft.