A control group in a scientific experiment is a group separated from the rest of the experiment, where the independent variable being tested cannot influence the results. This isolates the independent variable's effects on the experiment and can help rule out alternative explanations of the experimental results.
Control groups can also be separated into two other types: positive or negative.
Positive control groups are groups where the conditions of the experiment are set to guarantee a positive result. A positive control group can show the experiment is functioning properly as planned.
Negative control groups are groups where the conditions of the experiment are set to cause a negative outcome.
Control groups are not necessary for all scientific experiments. Controls are extremely useful where the experimental conditions are complex and difficult to isolate.
Example of a Negative Control Group
Negative control groups are particularly common in science fair experiments, to teach students how to identify the independent variable. A simple example of a control group can be seen in an experiment in which the researcher tests whether or not a new fertilizer has an effect on plant growth. The negative control group would be the set of plants grown without the fertilizer, but under the exact same conditions as the experimental group. The only difference between the experimental group would be whether or not the fertilizer was used.
There could be several experimental groups, differing in the concentration of fertilizer used, its method of application, etc. The null hypothesis would be that the fertilizer has no effect on plant growth. Then, if a difference is seen in the growth rate of the plants or the height of plants over time, a strong correlation between the fertilizer and growth would be established. Note the fertilizer could have a negative impact on growth rather than a positive impact. Or, for some reason, the plants might not grow at all. The negative control group helps establish that the experimental variable is the cause of atypical growth, rather than some other (possibly unforeseen) variable.
Example of a Positive Control Group
A positive control demonstrates an experiment is capable of producing a positive result. For example, let's say you are examining bacterial susceptibility to a drug. You might use a positive control to make sure the growth medium is capable of supporting any bacteria. You could culture bacteria known to carry the drug resistance marker, so they should be capable of surviving on a drug-treated medium. If these bacteria grow, you have a positive control that shows other drug-resistance bacteria should be capable of surviving the test.
The experiment could also include a negative control. You could plate bacteria known not to carry a drug resistance marker. These bacteria should be unable to grow on the drug-laced medium. If they do grow, you know there is a problem with the experiment.