To begin with, what are RCTs? These are experiments where people are given, at random, either an intervention (such as a drug) or a control and then followed up to see how they fare on various outcomes.

RCTs are conducted by researchers around the world. In the map, you can see how many RCTs have ever been published in high-ranked medical journals, by the country where the first author was based. Over 18,000 of these were from the United States, but most countries have had fewer than a hundred.1 RCTs have also become more common over time.2

It’s easy to take RCTs for granted, but these trials have transformed our understanding of cause and effect. They are a powerful tool to illuminate what is unknown or uncertain; to discern whether something works and how well it works.

But it’s also important to recognize that these trials are not always perfect, and understand why.