A randomized controlled trial, or RCT, is a specific kind of scientific experiment in which researchers screen and recruit people, then randomly assign them to a treatment or control group at the start of the study. In a double-blind trial, neither the researchers nor the study participants know which group they are in. This helps to reduce performance and treatment biases.
A placebo-controlled trial is one in which the control group takes a placebo, or a look-alike treatment that has no effect. High quality RCTs are considered to be the highest levels of scientific evidence, but they are also very expensive to stage and cannot always be done in an ethical way. For example, it’s unlikely that anyone would ever design a randomized controlled trial to test the effects of smoking on lung cancer, since assigning people to smoke would certainly harm their health.