I only have a small sample of pictures with a good comparison of in-game attendance of student section vs season ticket holders, but I'll give it my best using just USD from last year:
2010 vs USD
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Here's a couple more of the sidelines during the games without the student section to compare:
2009 vs SDSU
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2010 vs Northwestern (pre-game but seats were clearly 60-70% empty)
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2009 vs Purdue (student section only but check out the empty seats directly near it)
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I guess it's totally a judgment call but when I look back at picture comparisons I see a lot (percentally) of open seats in the regular seats and the student section mostly full (most students who buy tickets come, how long they stay is another question entirely).
As people always state (and I can attest to after sitting in the student section for a year for most of the games) upper deck seats being open is indicative of students packing 4-5 people for every 3 spots. Next game, count how many clearly empty chairback seats there are on the sidelines vs student section. It may not be a good test since so many Bison fans will be there, so maybe we wait til the B1G season?