Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Problem:Passage: One way to test a cause and effect hypothesis is to perform a controlled experiment. In a controlled experiment, two identical experiments are carried out side-by-side. In one of the experiments, the independent variable being tested is used, and in the other experiment, the control, the independent variable is not used. Answer: controlled experiment
Solution:
What type of experiment involves two identical experiments carried out side-by-side, with and without an independent variable?