Control Group Definition

In scientific experiments, the control group is the group of subject that receive no treatment or a standardized treatment. Without the control group, there would be nothing to compare the treatment group to. When statistics refer to something being “X times more likely to happen” they are referring to the difference in the measurement between the treatment and control group. The control group provides a baseline in the experiment. The variable that is being studied in the experiment is not changed or is limited to zero in the control group. This insures that the effects of the variable are being studied. Most experiments try to add the variable back in increments to different treatment groups, to really begin to discern the effects of the variable in the system.

Ideally, the control group is subject to the same exact conditions as the treatment groups. This insures that only the effects produced by the variable are being measured. In a study of plants, for instance, all the plants would ideally be in the same room, with the same light and air conditions. In biological studies, it is also important that the organisms in the treatment and control groups come from the same population. Ideally, the organisms would all be clones of each other, to reduce genetic differences. This is the case in many artificially selected lab species, which have been selected to be very similar to each other. This ensures that the results obtained are valid.

Examples of Control Group

Testing Enzyme Strength

In a simple biological lab experiment, students can test the effects of different concentrations of enzyme. The student can prepare a stock solution of enzyme by spitting into a beaker. Human spit contains the enzyme amylase, which breaks down starches. The concentration of enzyme can be varied by dividing the stock solution and adding in various amounts of water. Once various solutions of different strength enzyme have been produced, the experiment can begin.

In several treatment beakers are placed the following ingredients: starch, iodine, and the different solutions of enzyme. In the control group, a beaker is filled with starch and iodine, but no enzyme. When iodine is in the presence of starch, it turns black. As the enzyme depletes the starch in each beaker, the solution clears up and is a lighter yellow or brown color. In this way, the student can tell how long the enzymes in each beaker take to completely process the same amount of substrate. The control group is important because it will tell the student if the starch breaks down without the enzyme, which it will, given enough time.

Testing Drugs and the Placebo Effect

When drugs are tested on humans, control groups are also used. Although control groups were just considered good science, they have found an interesting phenomena in drug trials. Oftentimes, control groups in drug trials consist of people who also have the disease or ailment, but who don’t receive the medicine being tested. Instead, to keep the control group the same as the treatment groups, the patients in the control group are also given a pill. This is a sugar pill usually and contains no medicine. This practice of having a control group is important for drug trial, because it validates the results obtained. However, the control groups have also demonstrated an interesting effect, known as the placebo effect

In some drug trials, where the control group is given a fake medicine, patients start to see results. Scientists call this the placebo effect, and as of yet it is mostly unexplained. Some scientists have suggested that people get better simply because they believed they were going to get better, but this theory remains untested. Other scientists claim that unknown variables in the experiment caused the patients to get better. This theory remains unproven, as well.

Related Biology Terms

  • Treatment Group – The group that receives the variable, or altered amounts of the variable.
  • Variable – The part of the experiment being studied which is changed, or altered, throughout the experiment.
  • Scientific Method – The steps scientist follow to ensure their results are valid and reproducible.
  • Placebo Effect – A phenomenon when patients in the control group experience the same effects as those in the treatment group, though no treatment was given.


1. A scientist is studying the effect of a toxin on bacteria cells. The scientist divides a single population of bacteria into three parts. The parts are separated into different petri dishes and solutions of different strengths (5M and 10M) are applied to two dishes, while the third contains only bacteria. Which of these populations represents the control group?
A. The colony with 5M solution
B. The colony with 10M solution
C. The colony with no solution

Answer to Question #1

2. In an experiment testing the effects of strawberries on the health of rats, which of these would be a good control group?
A. A group of rats that gets only strawberries.
B. A group of rats that gets no strawberries in addition to a normal diet.
C. A group of rats that gets some strawberries, and a normal diet.

Answer to Question #2

3. Sometimes, the variable cannot be removed from the control group, and it must be standardized. A population of mice has an average level of protein in their blood of 10. Increased or decreased protein levels affect the functions mice are able to perform. Which of these would be the best control group?
A. 10 mice, all with levels of protein at 10.
B. 40 mice, all with levels of protein at 10.
C. 40 mice, all with levels of protein of 20.

Answer to Question #3

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