Kyle Allen and Patrick Mahomes are at opposite ends of the quarterback spectrum. Allen was an undrafted free agent; Mahomes is one of the most talented quarterbacks in league history and was the 2018 NFL MVP.

Allen ranks 32nd out of 34 qualifying quarterbacks in the most basic (in a good way) of passing stats: net yards per attempt.

Mahomes ranks 1st this year in NY/A, after ranking 1st in the same stat last year among quarterbacks who started at least 8 games.  Net yards per attempt is a good stat, and Mahomes is excellent at it because he’s an excellent quarterback (or maybe vice versa).

But you know better than to expect this to be a “Mahomes good Allen bad” post. Because I did a triple take this morning when I noticed that Kyle Allen has thrown for first downs at a higher rate this season than Mahomes.  That seemed impossible, and I had to double check twice just to make sure the data wasn’t wrong.

In general, there is a significant correlation between Net Yards per Attempt (which is passing yards, net of sack yards lost, divided by pass attempts plus sacks) and Passing 1st Down Rate (which is passing first downs divided by pass attempts plus sacks).   Both of these are very good stats to measure quarterback play, and last year, Mahomes led the NFL with a 43.2% passing first down rate.   Passing 1st Down Percentage is a good quick and dirty stat, and one where the best quarterbacks tend to fare very well. It is certainly not biased against a player like Mahomes.  But this year, Mahomes ranks 13th in that metric despite still having a very good NY/A average, while Allen shockingly ranks 11th in the metric.

So we have two pretty good, and easy to calculate passing stats, that in general are very correlated.  How correlated? Take a look at the graph below, which shows the same data as the table above.  And while the logos are for teams, the data  is for individual quarterbacks, not team-level data. So the Jets logo is only Sam Darnold, not the full Jets passing stats in 2019. And for the Redskins, Titans, and Steelers, it’s Dwayne Haskins, Ryan Tannehill, and Mason Rudolph in the chart below.  The Panthers, and to a lesser extent, the Chiefs, stand out as a notable outlier:

So, uh, what is going on here? Why in the world is Kyle Allen breaking this graph? As best I can tell, it’s the result of a few things.

1) Whether it’s due to his style of play or just randomness over a small sample, Kyle Allen takes a lot of long sacks. His average sack has been for -8.7 yards, the worst in the league. The average sack this season has lost 6.8 yards. For purposes of calculating Passing First Down Rate, all non-first downs are treated as equally bad. On the other hand, NY/A counts each yard, so that is driving some of the disconnect here.

2) Allen also takes a lot of sacks, while Mahomes does not. Think of it this way: both players pick up a first down on about 35% of dropbacks. On the remaining dropbacks, Passing 1st Down Percentage treats those 65% of plays as being equal. But Allen takes a sack on 14% of his dropbacks that don’t get a first down, while Mahomes takes a sack on only 6% of his dropbacks that don’t gain a first down. That has no impact on the player’s Passing 1st Down Percentage, but it does on his NY/A average. Allen is averaging a league-worst 0.4 NY/A on pass plays that do not pick up a first down. No quarterback has very good stats on pass plays that don’t pick up a first down, but Allen has the worst rate here because he takes a lot of sacks and they lose a lot of yards (he also has a very bad completion percentage on pass plays that don’t pick up a first down; all of these factors are why his NY/A on passes that don’t gain a first down is the worst in football).

3) On the other hand, it’s not all bad for Allen.  Tampa Bay QB Jameis Winston, as regular readers would expect, has the highest rate of completed passes that go for a first down at 64.3%. Winston always does well in this stat, as he throws the ball downfield and rarely checks down. But my guess is you would be shocked to know that Allen — who many think frequently checks down often to Christian McCaffrey — is right behind Winston at 62.9%. That’s right: Kyle Allen has the second highest rate of completions that go for a first down in the NFL.

Yes, he throws a lot to McCaffrey, and Carolina QB passes to Christian McCaffrey have only gained a first down on 52% of completions, but that’s actually a high number for a running back.  Meanwhile,  D.J. Moore has picked up a first down 73% of his completions, and Panthers passes completed to Greg Olsen and Curtis Samuel have gained a first down 69% and 68% of the time, respectively.

If you were to break down a pass play into four buckets — sack, incomplete, completion but not for a first down, completion for a first down — the disparity between Allen and Mahomes comes into focus. For Passing First Down %, the only thing that matters is the last bucket: a sack, an incomplete pass, and a completion that doesn’t pick up a first down are all treated as bad plays (this is why Passing First Down %, while a simple and good stat, is still a very basic stat: it misses a lot). Meanwhile, NY/A looks at all of the yards, but it doesn’t give special credit for picking up a first down: it combines the last two buckets completely, which is also bad.

When it comes to sack percentage, Mahomes is outstanding, while Allen is terrible: in addition, Allen also takes a lot of loooong sacks, and he really stands out historically in this regard. (Note: I make no claim as to whether this is due to lack of skill or randomness.)

When it comes to throwing incomplete passes, Allen and Mahomes are both generally in middle of the pack.  Mahomes throws fewer incompletions, but there is nothing nuanced to analyze here.

When it comes to completing passes that do not pick up a first down, Mahomes has a relatively high percentage here (28%) while Allen is extremely low at 21% (only Winston, at 20%, is lower).  This stat is not necessarily inherently good or bad: a lot of check downs that don’t do much but help a passer’s completion percentage are picked up here, but so are 8-yard passes on 1st down.

When it comes to completing passes for a first down, Allen ever so slightly edges Mahomes.  That’s the shocking part, and it’s because of the truly shocking reality that Allen has the 2nd highest rate of picking up first downs on completed passes.

Pretty interesting.  Let me know what you think in the comments.