Control Group: Definition & Explanation

A control group is used in an experiment to establish its validity. It is the group to which no treatment is administered. Learn more about how a control group works in this lesson.
What Is a Control Group?
Imagine that you want to know if salt makes water boil faster. You would want to heat one pan of water without salt and one pan of water with salt at the same temperature. Then you would compare how long it takes each pan of water to boil. In this experiment, the water without salt is your control group.

A control group is the group in a study that does not include the thing being tested and is used as a benchmark to measure the results of the other group.

Importance of Control Groups
There must be at least two groups in any valid experiment: the experimental group and the control group. The experimental group is the group in which you are testing something. For the experiment described earlier, the pan of water with salt added is the experimental group. The only difference between the two groups is the addition of the salt. This means that salt is the variable. A variable is the condition that is allowed to change.

In order for you to know exactly what causes a difference in the results between groups, only one variable can be measured at a time. You would compare the results from the experimental group with the results of the control group to see what happens when you change the variable you want to examine. A control group is an essential part of an experiment because it allows you to eliminate and isolate these variables.

Control groups are particularly important in social sciences, such as psychology. This is because it is practically impossible to completely eliminate all of the bias and outside influence that could alter the results of the experiment, but control groups can be used to focus on the variable you’re trying to test. Failure to provide evidence of strong control groups can cause a study to be considered invalid.

Types of Control Groups
There are two main types of control groups: positive control groups and negative control groups.

Positive Control Groups
In a positive control group, the control group is designed to produce the effect you are trying to reproduce in the experimental group.

This type of control group allows you to show that the desired result is possible in the experiment. In this situation, you would use something that is known to produce the desired result as the control group. This proves that the experimental conditions are valid. If the control group also fails, it would show that something is wrong with the conditions of the experiment. Positive control groups reduce the chance of a false negative. A false negative is a result that appears negative when it should not.