I took the overall avg for each season and adjusted accordingly. Example - avg completion % was 59.0 in 2001 and 65.4 in 2021. Multiply Brady's % by 1.108. Imho, it's not unreasonable, but some would consider it Eau du Dino. I'm sure old Quant me could come up with something crazier, but might as well keep it simple. Looking back, it looks like post merger, Stabler in '76 then followed by Montana in '89 had the best adjusted completion percentages. For QB rating, Staubach in '71 followed by Stabler in '76 (then BJones in '76 then Montana in '89).
Check out the NFL Season By Season Passing Stats including league totals and averages and more on Pro-football-reference.com
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