The main reason that it's important to test only one variable at a time is so that

Your experiment has an accurate outcome. The only variable that should be changing is what you have already determined to be the independent variable. All other variables must be controlled (sometimes called constant) in order to preserve the accuracy of the experiment and ensure that the only thing impacted the change in the dependent variable is directly caused by the independent variable.

Long term continuous groundwater monitoring, especially with warning systems and anthropic activity monitors

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