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A experiment should only have one variable.
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To eliminate confounding variables, or variables that were not controlled and damaged the validity of the experiment by affecting the dependent and independent variable, the experimenter should plan ahead. They should run many checks before actually running an experiment.
ONE :)
Just one at a time
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.
It depends what kind of experiment you do. For some you just need one. For others you may change two variables. In most cases you only change one
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As few as possible.
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