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What is a Control Group?

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University Course Reader · STEM

Participants who received no coaching served as the control group against which improvements were measured.

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Overview

A control group is the set of participants who do not receive the thing being tested, giving researchers a baseline for comparison. Without one, there is often no way to tell whether the treatment caused the change or whether it would have happened anyway. It is what separates a study from a story.
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Overview

A control group is the group that does not get the thing being tested, so you have something to compare against. Without it you cannot tell whether the treatment worked or whether things would have improved on their own. It is the difference between a study and an anecdote. 😎

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Detail

The problem a control group solves is that things change on their own: people recover from colds untreated, students improve over a school year, sales rise every December. So a group that got the treatment and then improved has proved very little by itself. Run a second group treated identically except for the one thing being tested, and now you have a comparison, because whatever happens in that group is what would have happened anyway. Who goes into which group has to be decided at random, since if people choose for themselves the keenest and healthiest will pick the treatment. In medicine the control group is usually given a placebo, a dummy pill with no active ingredient, because people who believe they are being treated often do feel better. Neither the patients nor their doctors are told who got the real drug, so belief cannot do the drug's job. Some questions cannot be tested this way at all, since nobody can randomly assign people to smoke. Without a proper control group, researchers must be much more cautious about claiming that one thing caused another.
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Detail

Things improve on their own, and that is the whole problem. Colds clear up, students get better over a year, sales climb every December. So your treated group improved? Congratulations, you have proved nothing yet. Run a second group treated exactly the same except for the one thing you are testing, and now you have a baseline, because whatever happens over there would have happened regardless. Who goes where has to be random, or the keenest and healthiest pile into the treatment side and you end up measuring them, not the treatment. Medicine hands the control group a placebo, a fake pill with nothing in it, because believing you are being treated genuinely makes people feel better, and nobody is told who got the real thing. Some questions never get this treatment at all, since you cannot randomly assign people to smoke, and without a real control group nobody can say with much confidence that one thing caused the other. 😎

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Analogy

Baking the same recipe twice, changing only the oven temperature. Swap the flour too, use a different tin, bake on a humid day, and the second loaf will come out different with no way to know which change did it. The second loaf is not there to be eaten. It is there to be compared.
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Analogy

Two identical houseplants on the same windowsill, one getting the fancy new plant food. If the fed one shoots up faster while the other just plods along, you have learned something real. Skip the second plant and all you have is a plant that grew, which is what plants do anyway, fed or not. The boring plant is the entire point.
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Formal definition — The same term, explained the usual way

A control group is a comparison group that does not receive the intervention under study, providing a counterfactual baseline against which treatment effects are measured. Random assignment is required to prevent systematic differences between groups, and blinding with placebo controls limits expectancy effects; where randomization is infeasible or unethical, observational designs substitute statistical adjustment and carry weaker causal claims.

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