A randomized trial is a trial in which participants are randomly allocated to one of a number of groups, each of which receives a different intervention or treatment. Researchers then compare the outcomes in the different groups to determine whether the intervention is effective. Randomized trials are often placebo-controlled, meaning that at least one group receives a fake treatment. In a properly placebo-controlled trial, none of the participants are made aware of which treatment they received. Where possible, such trials are also double-blind, meaning that neither the recipient nor the person giving the treatment knows whether the treatment is real or fake.
Because participants are randomly allocated into their respective groups, the composition of the population of each group should be very similar on average – the only significant difference between the groups should be the intervention they are receiving. This eliminates selection bias, which which is when the selection criteria for the study result in groups that differ from each other in ways besides the variable that the researchers are interested in. Randomization means that if researchers observe a difference in outcome between two groups, it is likely to be caused by their intervention and not some other factor. Correlation in a randomized study is therefore more likely to be causal compared with an observational study design.
Randomized, placebo-controlled (and preferably double-blind) trials (RCTs) are the gold standard of study designs, because they are able to establish causation, and are able to rule out the placebo effect as a potential cause of any observed correlation. RCTs are more expensive and harder to recruit for than observational studies, and usually have shorter durations. RCTs are also not always ethical or practical – it’s not ethical to randomize people to smoke cigarettes, for example, and it’s not possible to make a trial of exercise placebo-controlled.