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What are randomized controlled trials?

Hassaan Waqar

A randomized controlled trial, or RCT, is a study design used to randomly assign individuals from a target population into experimental and control groups. RCTs are used to prevent biases within samples that can lead to misinterpretation of experimental results.

Participants of an RCT are divided into two groups:

  • Experimental group
  • Control group

Experimental group

Participants belonging to the experimental group receive the variable that is being tested.

Control group

Participants belonging to the control group do not receive the variable that is being tested.

Sometimes, a placebo is given to the participants of the control group. A placebo appears to be similar to the variable that is being tested, but does not have any effect. This is done to eliminate the “placebo effect”, which can introduce bias in the experiment, as a group that does not receive treatment may act differently from the one that does.

For RCTs to be successful, participants should be divided randomly so that no difference exists between both groups. The only difference is the treatment that is being administered.


Randomized controlled experiments have been widely used in clinical research. For example, consider the following scenario:

A group of researchers wants to examine the effects of a new drug. They randomly choose and divide participants into two groups: the experimental and the control group.

The experimental group is treated with the drug that is being tested, while the control group is given a placebo. In this case, a placebo can be a sugar pill that has no effect.

After a period of time, the effects of the drug are examined through the scientific method. In medical research, this can be done through medical tests. The difference between the experimental and the control group results is observed, analyzed, and reported.

RCTs assume that the difference between both groups is only due to the treatment administered. This is why groups need to be assigned completely randomly to eliminate any bias.

The illustration below summarizes the scenario:

A Randomized Controlled Trial


  • In an RCT, observations can be made about the effect on the sample space with and without the treatment.
  • Good randomization of participants will eliminate bias.
  • Results can be analyzed using statistical tools.


  • Expensive in terms of time, money, and resources.
  • Participants might not always agree to be part of a particular group. For example, while studying the effect of smoking, some participants might not be willing to smoke. Thus, this introduces ethical concerns.


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Hassaan Waqar
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