The 1991 Eagles had a very bad passing offense. Philadelphia was one of four teams to finish with an ANY/A below 4.00, wasting a legendary defense along the way. You may have already known that, but here’s something you may not have known: Jim McMahon was Philadelphia’s starting quarterback that season, and he had a good season, ranking 12th in ANY/A and 13th in passer rating.

So yes, Philadelphia’s starting quarterback was an above-average passer despite Philadelphia’s passing attack being terrible. How did that happen? Well, it’s pretty simple: McMahon was responsible for 61% of the Eagles pass attempts that year, but also 66% of the team’s completions, 71% of the passing yards, 71% of the passing touchdowns, and only 47% of the sacks and just 41% of the interceptions. McMahon averaged 5.59 ANY/A; the rest of the Eagles passers averaged 0.77 ANY/A! That’s not a typo:

Thought of another way, non-McMahon passers had 226 dropbacks.  Using McMahon’s average of 5.59 ANY/A, we would “expect” McMahon to have produced 1,263 Adjusted Net Yards on those dropbacks.  In reality, other Eagles passers produced just 175 Adjusted Net Yards on those dropbacks, in large part due to 16 interceptions on 202 pass attempts.  This means McMahon “would have” produced 1,088 more Adjusted Net Yards than his backups.

That’s… a lot.  In fact, it’s the second-most in history using the following methodology:

• 1) Calculate the ANY/A for each passer on each team.  So for 1991 McMahon, it’s 5.59.
• 2) Calculate the ANY/A for the rest of that team’s passers for each passer.  For 1991 McMahon, it’s 0.77.
• 3) Subtract the result in step 2 from the number in step 1. This leaves us with 4.82 in the case of ’91 McMahon.
• 4) Multiply the result in step 3 by the smaller number of (a) that passer’s number of dropbacks and (b) the total number of dropbacks by the rest of the team.  In ’91 McMahon’s case, we use (b), which is 226, to get a result of 1,088.

The biggest difference in NFL history wasn’t McMahon, but Dan Fouts on the 1983 Chargers.   He averaged 7.32 ANY/A and threw 347 passes.  His backup, Ed Luther, averaged 3.77 ANY/A and threw 287 passes.  Fouts threw 20 TDs and 15 INTs, while Luther had 7 TDs and 17 INTs!  Non-Fouts passers in 1983 on San Diego had 308 dropbacks and averaged 3.66 ANY/A, exactly half of Fouts’ average!  Therefore, Fouts had 1,130 Adjusted Net Yards above expectation that season.

Three players from 2017 make the top 75 using this methodology, and my hunch is you could guess them pretty easily.  Then again, this formula isn’t supposed to shock you: it’s just a way of measuring which teams had a really good passer play about half a season, and really bad passers the rest of the season.