Randomized Controlled Trial
What is a Randomized Controlled Trial?
A Randomized Controlled Trial (RCT) is a scientific study that evaluates the effectiveness of an intervention by randomly assigning participants from an eligible population into either a treatment group that receives the intervention or a control group that does not.
The Basic Idea
Imagine that you wanted to know how effective an antidepressant is. While you could give the medication to an entire group of people who experience depression, it would be difficult to accurately measure its effectiveness because you wouldn’t be able to compare the results with a group of people who hadn’t taken the medication.
As such, you may design a clinical experiment where one group of people receive the antidepressant, and the other group—the control group—would not. In a randomized controlled trial, participants are randomly selected to be in the treatment (or experimental) group or the control group to avoid selection bias. By comparing the outcomes of the treatment group to the control group, you would be able to assess the effectiveness of the antidepressant more accurately.
Randomization
The hope in a randomized controlled trial is that the only significant difference between the people in the control and treatment groups is whether they receive the treatment or intervention being studied. Due to the random allocation of people into the groups, participant characteristics such as age, race, and gender are usually balanced, which allows researchers to attribute any difference in outcome to the intervention.
There are a few different ways to randomize participants:1
- Simple randomization: the most straightforward way to randomize people into groups, using tools like random number generators to assign all participants
- Block randomization: assigning participants into blocks first, and then assigning each block to a group. If you have 100 participants, you could divide them into four blocks of 25 each, then assign blocks 1 and 3 to the treatment group and blocks 2 and 4 to the control group.
- Stratified Randomization: assigning participants into different groups based on characteristics that could affect the results. For example, if you want an equal number of men and women in your treatment and control group, you would use sex to sort participants—assigning an even amount to each group
Randomization is done with the aim of balancing known and unknown confounders across groups, but it's not guaranteed that all characteristics will be perfectly balanced, especially in smaller trials.
Randomized controlled trials are the most rigorous way of determining whether a cause-effect relation exists between treatment and outcome and for assessing the cost effectiveness of a treatment.
– Bonnie Sibbald and Martin Roland, researchers for the National Primary Care Research and Development Centre at the University of Manchester, in their 1998 paper Understanding controlled trials: Why are randomised controlled trials important?2
About the Author
Emilie Rose Jones
Emilie currently works in Marketing & Communications for a non-profit organization based in Toronto, Ontario. She completed her Masters of English Literature at UBC in 2021, where she focused on Indigenous and Canadian Literature. Emilie has a passion for writing and behavioural psychology and is always looking for opportunities to make knowledge more accessible.
