Answer:
A controlled experiment is a scientific test done under controlled conditions, meaning that just one (or a few) factors are changed at a time, while all others are kept constant.
Step-by-step explanation:
A good example would be an experiment to test drug effects. The sample receiving the drug would be the experimental group while the sample receiving a placebo would be the control group. While all variables are kept similar (e.g. age, sex, etc.) the only difference between the groups is the taking of medication.