How Is Historical Interest Rate Data Calculated?

RateLadder had posted a note that Prosper has exceeded $40 Million in loans (go Prosper) when they did posted the Marketplace data. This lead to an interesting question: does the historical interest rate information reflect borrower rates or lender rates? This matters because it tells whether groups are lowering the borrower's interest rate (as Prosper intends) or merely redistributing the interest.

I started chewing on the published loan data, and I can't seem to figure out how Prosper got their historical interest rate information. I took a look at the NC credit group since it's a smaller group. Just looking at data for NC loans with groups, I took the average loan interest rate and a dollar-weighted interest rate and compared it to the Prosper published info.

• Prosper Group'ed NC: 17.676%
• My Group'ed NC, Loan Average: 21.46% (Borrower), 20.28% (Lender
• My Group'ed NC, Dollar-Weighted Average: 22.44% (Borrower), 21.24% (Lender)

This is a wildly different result, not just a rounding issue. I also noticed that there are NC loans in the $5k - $10k region even though Prosper does not publish historical information on these loans.

So, how is Prosper calculating their historical interest rates?