In science, when experimenting, it must be a controlled experiment. The Scientists must contrast an “experimental group” with a “control group”. The two groups are treated exactly alike except for one variable being tested.

For example, in an experiment to check the necessity of CO₂ for photosynthesis one can contrast the control group (a plant with freely available carbon dioxide) with an experimental group (a plant with no available carbon dioxide). The necessity of carbon dioxide would be proved when photosynthesis occurs in the control group and doesn’t occur in the experimental group.