When two comparisons groups within an experiment are identical except for one variable the experiment is called

Controlled experiment

Explanation:

A controlled experiment is a type of experimentation that is done under controlled conditions. The procedure entails changing or manipulating one (sometimes more than one) variable or factor at a time, but keeping everything else the same between the groups.

This is done to ensure that the factor or variable they are testing is what elicits any changes in the subject. One group is always kept in normal or standard conditions, to serve as a base line to compare to.