This is another graph that caught my eye from the study posted by Harvard and Boston University researchers:

This bears out what I’ve been hearing from Groupon insiders.

In the latest amendment to the S-1, Groupon also reported that its revenue per sales rep declined from $172,000 in 1Q2011 to $138,000 in 2Q2011.

Groupon’s model is heavily dependent on its salesforce. To the extent that each deal brings in less revenue, selling expenses alone will become an even bigger problem for the company.