What is the most difficult part of a controlled experiment?

Testing multiple variables is the most difficult part of a controlled experiment.

Explanation:

An experiment in which one or some of the factors are varying with respect to time and while the other factors are maintained constant is known as a controlled experiment. More the number of testable variables more will be the issue in obtaining a conclusion, at the same time what would be the factors that should be kept constant will also become a problem. Thus, using multiple variables for examining can become tough in a controlled experiment.

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