I had a conversation yesterday with a PhD student friend (also in international health) about the evaluation of microcredit programs. I was trying to summarize -- off the top of my head, never a good idea! -- recent findings, and wasn't able to communicate much. But I did note that like many aid and development programs, you get a pretty rosy picture when you're using case studies or cherry-picked before-and-after evaluations without comparison groups. So I was trying to describe what it looks like to do rigorous impact evaluations that account for the selection biases you get if you're just comparing people who self-select for taking out loans versus controls. After that discussion, I was quite happy to come across this new resource on David Roodman's blog: yesterday DFID released a literature review of microfinance impacts in Africa.
On a related note, Innovations for Poverty Action hosted a conference on microfinance evaluation last October, and many of the presentations and papers presented are available here. The "What Are We Learning About Impacts?" sections includes presentations given by Abhijit Banerjee (PDF) and Dean Karlan (PDF) of Yale. Worth reading.