A control group in science is a group of people that closely resemble a treatment group, but do not receive the same treatment as the other group. This helps scientists compare those people who received the treatment against those who did not.
Control groups are often used when running experiments on new or different drugs or treatments. Control groups allow researchers to study these groups and compare or measure one variable at a time. The control group, or the group unaffected by the treatment, gives scientists a baseline with which to compare their results.
In some cases, a control group is given a placebo while the other group is given the real drug. Neither group knows if it is the one to receive the real medication and this type of research rules out the placebo effect. This type of study is called a "double-blind" study. In the double-blind study, not even the doctors know which group received the placebo.
Scientific control groups do not have to consist of people. For example, a doctor working on a new strain of antibiotics can use samples of bacteria or a well-established antibiotic as a control group. If all the samples fail to provide the desired results, something is wrong with the experiment and the variables will need to be changed.