A randomized controlled trial (RCT) is an experimental study in which participants are randomly assigned to different groups to compare the effects of an intervention.
One group typically receives the intervention being studied, while another receives a comparison such as a placebo, standard treatment, alternative intervention, or no intervention.
RCTs are commonly used in medicine, psychology, education, public health, and other fields that evaluate the effects of treatments, programs, or interventions.
How do randomized controlled trials work?
Researchers randomly assign participants to study groups to help balance known and unknown differences between the groups. This makes it more likely that differences in outcomes are due to the intervention rather than other factors.
An RCT may involve:
Randomly assigning participants to two or more groups
Giving each group a different treatment, intervention, or condition
Using a control or comparison group as a reference
Measuring outcomes using the same methods across groups
Comparing outcomes to determine whether the intervention had an effect
Blinding participants or researchers to group assignments when possible
For example, researchers might randomly assign patients to receive either a new treatment or standard care, compare two teaching methods across groups of students, or test whether a behavioral program improves a specific outcome.
Strengths of randomized controlled trials
RCTs allow researchers to:
Reduce differences between groups through random assignment
Compare interventions under controlled conditions
Reduce the risk of selection bias
More confidently assess whether an intervention caused an observed effect
Use control or comparison groups to provide a clear reference point
Randomization makes RCTs especially useful for studying cause-and-effect relationships.
In clinical and biomedical research, well-designed RCTs are often considered one of the most reliable methods for evaluating whether an intervention causes a particular outcome. As a result, they are commonly placed high in evidence hierarchies.
Limitations of randomized controlled trials
RCTs can be expensive, time-consuming, or difficult to conduct. In some situations, randomly assigning participants to an intervention may also be impractical or unethical.
Results may not always reflect what happens in broader real-world populations, particularly when studies use strict eligibility criteria or highly controlled conditions.
The reliability of an RCT also depends on factors such as sample size, participant retention, adherence to the intervention, outcome measurement, and how well the study was designed and conducted.