There are three types of variables tested: manipulated variables, controlled variables, and experimental variables.
It depends on the experiment. Normally only one is tested at a time because they can affect the experiment. Variables are tested in a controlled experiment to see whether they affect the outcome and also how.
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In a controlled experiment only 1 variable is being tested.
The trick to designing a good experiment is to figure out a way for it to test the effects of only one variable, and to avoid any effects of others.
It depends on the experiment. Normally only one is tested at a time because they can affect the experiment. Variables are tested in a controlled experiment to see whether they affect the outcome and also how.
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Only the one being tested
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In a controlled experiment only 1 variable is being tested.
Just one at a time
The trick to designing a good experiment is to figure out a way for it to test the effects of only one variable, and to avoid any effects of others.
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An experiment of any kind can have infinitely many variables. A controlled experiment can have just as many, provided that all but one are kept exactly the same.
All variables except one, the experimental variable, are kept constant in an experiment.
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