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What is a Control Group?

Tricia Christensen
Updated May 21, 2024
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How does science really know whether something is effective or not? Scientific inquiry answered this question long ago by trying to create specific controls in experiments. One such measure is the control group, or a group of similar people (animals, plants, etc) that are observed undergoing the same conditions as the test group, without receiving the thing the scientist wants to study. By creating as similar conditions as possible for a test and a control group, the scientist can determine the true effects of something he wants to test, by eliminating false results, especially if the only substantial variation is either receiving or not receiving the thing studied.

It’s easiest to think of the control group in terms of human studies, especially those that involve drug research. In a number of what are called double-blind studies, people participate without knowing whether they’re receiving a new drug. The other group gets a placebo and won’t know the results of the study until much later, if ever.

These clinical trials can reveal much about how effective a treatment might be, and they also show when it doesn’t work. For instance a control group not receiving the drug that reports improved symptoms in a much higher percentage than those reported by the group getting the drug would suggest the drug might not be as effective as hoped. Not only can potential effectiveness of medications be tested this way, but these studies can also show any side effects or long-term problems that might result.

Another way in which control groups can be formed is not quite as accurate and is called historical control. In this scenario the group is actually people who participated in an experiment in the past, selected as a way to compare results with a group that is currently being studied. Historical control is not always as accurate because theoretically a control group should be as greatly similar to the test group as possible.

In other words, a test can’t use samples that are as like, because the control group will be very different than the present group. Also, in controlled studies, part of the way that people try to get accurate results is by trying to make environment the same or similar for all participants. This can’t be done with a group studied in the past that forms the control, though a scientist may definitely look for previous control groups that have similarities to the people or other things currently being studied.

In all human groups, especially in lengthy experiments, it’s difficult to create control groups that are exactly similar to non-control groups, but scientists do have ways of narrowing down what they want. Those who participate in research may undergo lengthy physical examinations and fill out multiple questionnaires to make sure they are as similar to everyone else in the group being tested. Scientists might exclude participants who don’t show this similarity because additional factors they possess could skew results of testing, rendering the control group not as effective.

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Tricia Christensen
By Tricia Christensen
With a Literature degree from Sonoma State University and years of experience as a All The Science contributor, Tricia Christensen is based in Northern California and brings a wealth of knowledge and passion to her writing. Her wide-ranging interests include reading, writing, medicine, art, film, history, politics, ethics, and religion, all of which she incorporates into her informative articles. Tricia is currently working on her first novel.
Discussion Comments
By browncoat — On Dec 11, 2013

@umbra21 - I think the idea of a control group is a valuable one in almost any case. It is a relatively fundamental concept to have a control group vs an experimental group even if it doesn't always look exactly like a traditional science experiment.

But there are definitely famous experiments which didn't use this kind of control and they really should have.

The one that springs to mind is the Standford Prison experiment, where they took groups of men and arbitrarily assigned them as prisoners and guards and watched how they changed psychologically. They always say it proves something about the nature of power, but it was a very limited experiment. The men were similar in age and background, not to mention the same gender, and there was no group in similar conditions without the same power structure to compare what would happen. Without that kind of control group, we don't know how much was due to the power imbalance and how much was due to other factors.

By umbra21 — On Dec 10, 2013

@Mor - The scientific method introduced control groups to human inquiry and they are definitely valuable, but they aren't always completely necessary. I mean, in your example, the community garden might have benefits that were not immediately obvious, and could only be seen with a control group, but there are a lot of other factors to take into account, like whether those benefits are worth the cost and whether any communities can really be compared like that.

By Mor — On Dec 09, 2013

It's so important to teach children about control groups and why we need them, because this is an idea that really affects a lot of decisions in life. When you think about it, stuff like politics and community decisions would often be better for everyone if they were made based around real evidence.

But they are usually based around evidence that was found with no control group to verify it.

For example, people might decide that a local community garden is too expensive to keep and doesn't add anything to the area, and so they shut it down. But since they have no evidence of what might happen to the area without the garden they are basically making the decision based on flawed evidence.

If they wanted to really discover what the best thing to do would be they would investigate similar places with and without gardens and compare the standards of living or whatever benchmark they need.

Tricia Christensen
Tricia Christensen
With a Literature degree from Sonoma State University and years of experience as a All The Science contributor, Tricia...
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