I think it's very difficult to answer this question simply. Depending on the sophistication of your research model and the amount of data and the accuracy with which the research is carried out, you can test several variables simultaneously. This can be done 'legitimately', and sometimes it is too costly to run research projects that test one or two variables at a time. Some of these complex and sophisticated statistical models are used very infrequently, but it can be done.
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.
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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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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There are many different things that are measured and tested by the data collected in an experiment. Color changes are one thing that can be measured and tested for example.
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.