Ronald Aylmer Fisher was a statistician and eugenicist who developed the idea of randomized controlled trials (RCTs) to estimate causal effects. His and other early statisticians’ support for eugenics has given statistics a bad reputation. However, his ideas about RCTs have saved lives all over the planet. RCTs are the last hurdle that any new pharmaceutical product must pass before it can be approved for use. In the social sciences, we have also started to use RCTs more and more in recent decades.

The basic idea of an RCT is to run an experiment with a treatment group and a control group. The participants are randomly assigned to either the treatment group or the control group. This ensures that the two groups are as similar as possible at the start of the experiment, except for the fact that the treatment group receives the treatment and the control group does not. This randomization is what makes RCTs the gold standard for estimating causal effects. It is the only way to be sure that any difference between the two groups at the end of the experiment is due to the treatment, and not to some other factor that may have influenced who was assigned to the treatment group and who was assigned to the control group.

RCTs are not always possible to conduct, but they are the best way to estimate causal effects when they are possible. They are the gold standard for a reason.

Lecture slides.

Reading: Angrist and Pischke (2014, Ch. 1: pages 12-32).

Optional (skim) reading: Schaeffer and Kas (2024).