A good experimental design ensures that scientists are testing what they want to test and the their results will be meaningful. When an experiment is done in a laboratory, all conditions can be kept constant, except form an experimental variable,which is deliberatery changed. One or more test groups are exposed to the experimental variable,but one other group, called the control group,is not. If, by chance, control group shows the same results as the test group, the experimenter knows the results do not support the hypothesis.