What is the control group? Definition and examples

The control group is the group in an experiment which is free of intervention by the researchers. The control group in a clinical trial, for example, comprises people who do not receive the medication that the researchers are studying.

This group contrasts with the ‘intervention group‘ or ‘treatment group.’

In an experiment, the control group either receives no treatment or gets the standard treatment.

According to BusinessDictionary.com, a control group is:



“A group of subjects or conditions that is matched as closely as possible with an experimental group, but is not exposed to any experimental treatment.”

“The results are then compared to determine the changes that may occur due to the experimental treatment.”

Control group – example

Let’s imagine that a fictitious company – ACME Therapy – has a new experimental drug. The company has given its new drug the name ‘AcheAway.’ AcheAway is for the treatment of splitting headaches.

Its researchers want to know how effective it is. So, they decide to carry out an experiment or trial that compares it with a placebo.

A placebo is a dummy drug. In other words, a pretend medicine that has no active ingredient. Perhaps it is made of flour with chalk. However, the human volunteers in the trail don’t know this.

Control group vs intervention group image
As you can see in this image, the difference in the results between the intervention and control groups tells us that the headache medication is effective. If this were a small trial, the pharmaceutical company would then organize a larger trial.

Three groups

The researchers recruit 80 volunteers. They all suffer from regular splitting headaches. In this trial, each time they have a headache, they receive a tablet. They do not know what the tablet has.



The doctors who give them the tablets do not know whether they are giving the volunteers a placebo, or AcheAway.

When neither the doctors nor the volunteers know what is in the tablets, it is a ‘double-blind trial.’

The trial organizers split up the 80 volunteers into two groups of forty:

The intervention group

The people in this group receive a tablet containing AcheAway every time they have a headache.

The control group

The volunteers in this group get a placebo tablet whenever they have a headache.



The trial results

The trial lasts twelve weeks. At the end of the period, we have the following statistics:

  • Of the 40 people in the intervention group, 34 (85%) said their tablet helped them. In other words, it reduced their headache symptoms.
  • Of the forty volunteers in the control group, just nine (22.5%) said their tablet reduced their headache symptoms.

By comparing the results of the intervention group with the control group, we now know that AcheAway is better than a placebo. In medicine or experiments, we say that there was a ‘statistically significant’ difference (There is a significant difference between 85% and 22.5%).

If a pharmaceutical company wants FDA approval for a new drug in the United States, it needs to carry out several trials, including clinical trials (with humans).  It then submits all the details of those trials to the FDA. FDA stands for Food and Drug Administration. The FDA is the US regulatory agency in charge of medications, tobacco products, foods, cosmetics, and medical devices.