The 90% figure is based on the amount of money repaid. More precisely, it's the amount that has been repaid for loans disbursed in 2014, divided by the amount that is due to have been repaid.

I expect the default rate for loans being issued today will be less than 10%, because some of the amounts still outstanding will likely be repaid late. 90% is the portion of due amounts that has already been repaid (for 2014 loans).

That said, we are continuously improving our lending model in response to experience, and the repayment rate is increasing with each new cohort of loans we disburse.

Bayes Impact (another YC nonprofit which connects data scientists to nonprofits) is developing a credit risk prediction algorithm for use in vetting Zidisha loan applicants. Once that is deployed, we expect it will improve repayment rates substantially.

Zidisha is working very hard to bring down the default rate. But as this is a relatively new approach, a lot of things can only be learned through experimentation. So far the results are encouraging and if this trend continues default rate should come down even further. You can read about some of the efforts here: http://venturebeat.com/2014/08/20/y-combinator-backed-zidish...

Hope it helps and do share any feedback/suggestions you have. Thanks!