I heard this question at an impact evaluation training event a few weeks ago. I’ve heard some variation on it many times. Wouldn’t it be grand if there were a magic number? “5 times. If it works 5 times, it will work anywhere.” Alas, ’tis not so.
But Mary Ann Bates and Rachel Glennerster have a good answer in their new essay in the Stanford Social Innovation Review:
Must an identical program or policy be replicated a specific number of times before it is scaled up? One of the most common questions we get asked is how many times a study needs to be replicated in different contexts before a decision maker can rely on evidence from other contexts. We think this is the wrong way to think about evidence. There are examples of the same program being tested at multiple sites: For example, a coordinated set of seven randomized trials of an intensive graduation program to support the ultra-poor in seven countries found positive impacts in the majority of cases. This type of evidence should be weighted highly in our decision making. But if we only draw on results from studies that have been replicated many times, we throw away a lot of potentially relevant information.