Question

Why do scientists change only one variable in a controlled experiment?

Answer 1

To look at the effect of changes in that variable on the outcome of the experiment.

Explanation:

If more than one variable is changed in an experiment, scientist cannot attribute the changes or differences in the results to one cause. By looking at and changing one variable at a time, the results can be directly attributed to the independent variable. Thus coming to the conclusion of the relationship between the variable and the results, whether the relationship is a correlation or causation.