Control vs. Experimental Group: How Do They Differ?

Chemistry lab
The variable is tested on the experimental group. nicolas_/Getty Images

In an experiment, data from an experimental group is compared with data from a control group. These two groups should be identical in every respect except one: The difference between a control group and an experimental group is that the independent variable is changed for the experimental group, but is held constant in the control group.

An experimental group is the group that receives an experimental procedure or a test sample.

This group is exposed to changes in the independent variable being tested. The values of the independent variable and the result on the dependent variable are recorded. An experiment may include multiple experimental groups at one time.

A control group is a group separated from the rest of the experiment such that the independent variable being tested cannot influence the results. This isolates the independent variable's effects on the experiment and can help rule out alternate explanations of the experimental results.

While all experiments have an experimental group, not all experiments require a control group. Controls are extremely useful where the experimental conditions are complex and difficult to isolate. Experiments that use control groups are called controlled experiments.

Control Groups and Placebos

The most common type of control group is one held at ordinary conditions so it doesn't experience a changing variable.

For example, If you want to explore the affect of salt on plant growth, the control group would be a set of plants not exposed to salt, while the experimental group would receive the salt treatment. If you want to test whether duration of light exposure affects fish reproduction, the control group would be exposed to a "normal" number of hours of light, while the duration would change for the experimental group.

Experiments involving human subjects can be much more complex. If you're testing whether a drug is effective or not, for example, members of a control group may expect they will not unaffected. To prevent skewing the results, a placebo may be used. A placebo is a substance that doesn't contain an active therapeutic agent. If a control group takes a placebo, participants don't know whether they are being treated or not, so they have the same expectations as members of the experimental group.

However, there is also the placebo effect to consider. Here, the recipient of the placebo experiences an effect or improvement because she believes there should be an effect. Another concern with a placebo is that it's not always easy to formulate one that truly free of active ingredients. For example, if a sugar pill is given as a placebo, there's a chance the sugar will affect the outcome of the experiment.

Positive and Negative Controls

Positive and negative controls are two other types of control groups:

Positive control groups are control groups in which the conditions guarantee a positive result. Positive control groups are effective to show the experiment is functioning as planned.

Negative control groups are control groups in which conditions produce a negative outcome.

Negative control groups help identify outside influences which may be present that were not unaccounted for, such as contaminants.