Wrong, passing ypg allowed is only one, of many statistics, that can be used. In FACT, most knowledgable football people understand Completion Percentage the far greater determinant of Passing Defense dominance (Especially when difference in passing YPG is like 10 or 15 yards.... but difference in Completion % is 50% versus 60%.... accompanied with 50% Completion % with 18 INTs vs 60% Completion % with only 9 INTs).
You're utterly full of $hit that the difference in schedules explain this. I don't care what schedule you play - 14 games, 100% D1A (i.e., FBS) and you have a 50% Completion % and 18 INTs is insanely impressive... and I don't care who you played! Good example is Indiana (who played on avg - and BEAT - absolutely no one, is #10 in FBS in Passing YPG Allowed (176 ypg), but allowed a 62.5% Comp %, which is Bottom Quartile in FBS! (also only 11 INTs for Indiana - well down the list).
PSU is a good example - as many have pointed out, PSU has played tons of bad b1g teams and a weak non-conference schedule.... did it greatly elevate our Defensive Passing Completion % of 60%??? The answer is, no it didn't. Allowing a 50% Comp % is insanely impressive.... and I don't care who you play especially if 100% of the teams are FBS. Using unbiased sources, ND's SOS is not that much different than PSU's or duhO$U's - and is infinitely better than Indiana's.