Randomized Controlled/Clinical trial

Meaning: An RCT is an experiment in which participants are randomly assigned to a treatment group or to a control group that gets a placebo, a standard treatment, or nothing. Chance assignment makes the groups similar on average, even on variables nobody measured, which reduces confounding. Blinding (participants, or both participants and researchers, unaware of group membership) limits placebo effects and experimenter bias. Because mainly the independent variable differs between groups, this design is the strongest for supporting cause and effect.

On the MCAT: Passages describe a study and ask what supports a causal claim, what confound could remain, or how to improve validity. Expect comparisons with observational designs (cohort, case-control, survey), which show association but are more open to confounding. Questions may also ask why a control group or blinding is needed, or what is wrong when participants choose their own group (self-selection).

Common confusion: Random assignment is not random sampling. Assignment decides who gets which condition and supports internal validity, meaning a causal conclusion within the study. Sampling decides who enters the study and supports external validity, meaning generalizability to a population. A trial can have one without the other. Also, "controlled" means there is a comparison group, not that every variable is held constant.