A good experimental design ensures that scientiets are testing what they want to tast and the their results will be meaningfal. 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 voriable,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.