What is a role of a control group?

Answer 1

In conducting a scientific experiment, a scientist should have a control group and an experiment/treatment group. The scientist proposes a null hypothesis (null because it is intended to be disproven). The scientist then treats the two groups identically except the treatment group receives the treatment and the control group does not. Because the two groups were treated identically except for the treatment, any subsequent differences in the groups is (tentatively) attributed to the treatment. Thus,the control group is the group NOT receiving the treatment. For example, the scientist could propose that "fish will not die if they stop receiving food" (as a null hypothesis). The scientist then places two fish tanks full of similar fish into the same room, so that temperature, air pressure and light are equal. Then the scientist feeds one tank the recommended amount of fish food, while not feeding the other tank at all. After one week of this treatment, the scientist observes the tank receiving food (the control group) contains live fish while the group receiving the treatment (starvation) only contains dead fish. The scientist would then disprove the null hypothesis and conclude (tentatively) that fish do need food to continue living.