Alleviating poverty is more guesswork than science, and lack of data on aid's impact raises questions about how to provide it. But Clark Medal-winner Esther Duflo says it's possible to know which development efforts help and which hurt -- by testing solutions with randomized trials.

The thing is, if we don't know whether we are doing any good, we are not any better than the Medieval doctors and their leeches. Sometimes the patient gets better, sometimes the patient dies. Is it the leeches? Is it something else? We don't know.

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