In the early 90s,Roland Preston when a young economist named Michael Kremer finished his PhD, there had been a few economic studies based on randomized trials. But they were rare. In part because randomized trials – in which you recruit two statistically identical groups, choose one of them to get a treatment, and then compare what happens to each group – are expensive, and they take a lot of time.

But then, by chance, Michael had the opportunity to run a randomized trial in Busia, Kenya. He helped a nonprofit test whether the aid they were giving to local schools helped the students. That study paved the way for more randomized trials, and for other economists to use the method.

On today's show, how Busia, Kenya, became the place where economists pioneered a more scientific way to study huge problems, from contaminated water to low graduation rates, to HIV transmission. And how that research changed government programs and aid efforts around the world.

This episode was produced by James Sneed with help from Willa Rubin. It was engineered by James Willetts. It was fact-checked by Sierra Juarez and Emma Peaslee. It was edited by Molly Messick. Jess Jiang is our acting executive producer.

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Music: "Smoke and Mirrors," "Slowmotio," and "Icy Boy."