This lesson explains the basics of a control group, including:
A control group is a group that doesn't receive any kind of treatment. Now, you might be wondering if that's just a waste to have a group that doesn't receive any treatment. Well, it's not a waste, because a group that doesn't receive any treatment helps to establish a baseline for what the effectiveness of no treatment would have been. And so this helps us to determine the effectiveness of the treatments.
Suppose you want to test the effectiveness of a new drug that has been discovered for AIDS. Some people might say that you should just give the drug to everybody, because it might help. And that's true. It might help, but it might not.
If you gave the drug to everybody, you wouldn't be sure if maybe some people just got better by chance. You can't be sure whether observed differences are due to the drug or something else.
So you would create a control group. This means that some people will receive the real drug, and some people will receive a fake drug. This allows you you to see if the differences are significant enough.
You can begin to imagine that if there is a group established that does not receive a drug or treatment of some kind, there will be a group that does. This group, the one that receives the drug, modified tests, video screening, etc., is called the treatment group.
Making comparisons between the two groups - control and treatment - is best if they're as similar as possible, except for the treatment that treatment group is receiving. The response of the control group will be compared with the response of the treatment group. And if there's a difference, then you can attribute it to the treatment, and not some other factor.
Sometimes it's possible to have multiple treatments compared to a single control group.
An experiment on plants is created to determine if colored lighting is just as good as regular old sunlight for plants.
The control group might consist of randomly assigning certain plants to be outside in the sun, and then randomly assigning other plants to be under one of three colored lights: blue, orange, green.
This design will test multiple treatment groups with one control group. The control group helps establish a baseline for growth to compare against all of the other groups.
The purpose of the control group is to provide a baseline, so it can be determined if there's a difference between the treatments. The baseline is set by assigning no treatment to the control group.
The control group receives no active treatment. The treatment group does receive an active treatment.
Source: Adapted from Sophia tutorial by Jonathan Osters.
A group which receives no treatment (or an inert treatment) in an experiment. This group's response provides a standard against which the response to the treatment(s) can be compared.
A group that is manipulated by the treatments in the experiment. There may be many treatment groups in an experiment, but only one control group.