What is Randomized controlled trial?
A randomized controlled trial (RCT) is an experimental design that randomly assigns participants to treatment and control conditions to isolate the causal effect of an intervention. It is the gold standard for causal inference because randomization eliminates confounding.
How it works
Random assignment ensures that the treatment and control groups are equivalent on all measured and unmeasured variables before the intervention, so any post-treatment difference can be attributed to the treatment. Key design elements include random allocation, blinding (participants and/or researchers do not know who received which treatment), control groups (no treatment, placebo, or active comparison), and adequate sample size (for statistical power). RCTs are not always feasible (ethical constraints, practical limitations) or necessary (some questions do not require causal evidence).
Applied example
A company testing whether a new onboarding email sequence increases customer retention randomly assigns new customers to receive either the new sequence (treatment) or the existing one (control) and compares retention rates after 90 days. Random assignment ensures that any difference in retention is caused by the email sequence, not by differences in the customers who received each version.
Why it matters
RCTs provide the strongest evidence for causal claims, but their strength depends on proper execution, and they must be complemented by other designs for questions where randomization is impractical or unethical.
