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The Difference Between Control Group and Experimental Group

By Julia Simkus, published Feb 22, 2022

In a controlled experiment scientists compare a control group and an experimental group that are identical in all respects, except for one difference - experimental manipulation.

Unlike the experimental group, the control group is not exposed to the independent variable under investigation and so provides a base line against which any changes in the experimental group can be compared.

Since experimental manipulation is the only difference between the experimental and control groups, we can be sure that any differences between the two are due to experimental manipulation rather than chance.

Almost all experimental studies are designed to include a control group and one or more experimental groups. In most cases, participants are randomly assigned to either a control group or an experimental group.

Because participants are randomly assigned to either group, we can assume that the groups are essentially identical with the exception of the manipulation of the independent variable in the experimental group.

It is important that every aspect of the experimental environment is the same and that the experimenters carry out the exact same procedures with both groups so researchers can confidently conclude that any differences between groups are actually due to the difference in treatments.

What is a Control Group?

A control group consists of participants who do not receive any experimental treatment. The control participants serve as a comparison group.

The control group is matched as closely as possible to the experimental group, including age, gender, social class, ethnicity etc.

The difference between the control group and the experimental group is that the control group is not exposed to the independent variable, that is thought to be the cause of the behavior being investigated.

Researchers will compare the individuals in the control group to those in the experimental group to isolate the independent variable and look at the impact it had.

The control group is important because it serves as a baseline, enabling researchers to see what sort of impact changes to the independent variable produce and strengthening researchers’ ability to draw conclusions from a study.

Without the presence of a control group, a researcher cannot determine whether a particular treatment truly has an effect on an experimental group.

Control groups are critical to the scientific method as they help ensure the internal validity of a study.

Example of a Control Group

Assume you want to test a new medication for ADHD. One group would receive the new medication and the other group would receive a pill that looks exactly the same as the one that the others received, but it would be a placebo. The group who takes the placebo would be the control group.

Types of Control Groups

  1. Positive Control Group
    • A positive control group is an experimental control that will produce a known response or a desired effect.
    • A positive control is used to ensure the success of a test and confirm the validity of an experiment.
    • For example, when testing for a new medication, an already commercially available medication could serve as the positive control.
  2. Negative Control Group
    • A negative control group is an experimental control that does not result in the desired outcome of the experiment.
    • A negative control is used to ensure that there is no response to the treatment and help identify the influence of external factors on the test.
    • An example of a negative control would be the use of a placebo when testing for a new medication.

What is an Experimental Group?

An experimental group consists of participants who are exposed to a particular manipulation of the independent variable. These are the participants who receive the treatment of interest.

Researchers will compare the responses of the experimental group to the responses of a control group to see if the independent variable had any impact on the participants.

There must be at least one control group and one experimental group in an experiment; however, a single experiment can include multiple experimental groups, which are all compared against the control group.

Having multiple experimental groups enables researchers to vary different levels of an experimental variable and compare the effects of these changes to the control group as well as among each other.

Example of an Experimental Group

Assume you want to study to determine if listening to different types of music can help with focus while studying.

You randomly assign participants to one of three groups: one group that listens to music with lyrics, one group who listens to music without lyrics, and another group that listens to no music.

The group of participants listening to no music while studying is the control group and the groups listening to music, whether with or without lyrics, are the two experimental groups.

Frequently Asked Questions:

1. What is the difference between a control group and an experimental group?

Put simply, an experimental group is the group that receives the variable, or treatment, that the researchers are testing whereas the control group does not. These two groups should be identical in all other aspects.

2. Do experimental studies always need a control group?

Not all experiments require a control group, but a true “controlled experiment” does require at least one control group. Experiments that use a within-subjects design, for example, do not have a control group.

In within-subjects designs, all participants experience every condition and are tested both before and after they are exposed to treatment.

These experimental designs tend to have weaker internal validity as it is more difficult for a researcher to be confident that the outcome was caused by the experimental treatment and not by a confounding variable.

3. Can a study include more than one control group?

Yes, studies can include multiple control groups. For example, if there are several distinct groups of subjects who do not receive the treatment, then these would be the control groups.

About the Author

Julia Simkus is an undergraduate student at Princeton University, majoring in Psychology. She plans to pursue a PhD in Clinical Psychology upon graduation from Princeton in 2023. Julia has co-authored two journal articles, one titled “Substance Use Disorders and Behavioral Addictions During the COVID-19 Pandemic and COVID-19-Related Restrictions," which was published in Frontiers in Psychiatry in April 2021 and the other titled “Food Addiction: Latest Insights on the Clinical Implications," to be published in Handbook of Substance Misuse and Addictions: From Biology to Public Health in early 2022.

How to reference this article:

Simkus, J. (2022, Feb 22). The Difference Between Control Group and Experimental Group. Simply Psychology. www.simplypsychology.org/control-and-experimental-group-differences.html

Sources

Bailey, R. A. (2008). Design of Comparative Experiments. Cambridge University Press. ISBN 978-0-521-68357-9.

Hinkelmann, Klaus; Kempthorne, Oscar (2008). Design and Analysis of Experiments, Volume I: Introduction to Experimental Design (2nd ed.). Wiley. ISBN 978-0-471-72756-9.

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