They shouldn't, ever. It defies the whole point of a controlled experiment.
no one know's these answers
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As few as possible.
As many as are necessary, as few as possible.
There are infinitely many possible ways in which two variables can be related to one another.
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1) I learned that you can have as many variables as you want. 2) You can change the variables. 3) Variables is used in an experiment. 4) A variable is something that can be changed, such as a characteristic or value. Variables are generally used in Psychology experiments to determine if changes to one thing result in changes to another.
There should be one dependent variables. Depending on the type of research you are doing, the amount of independent variables will change. If you are doing research on a large scale, you will use more independent variables. If it's on a small scale, you will use very little. If you are not able to run your regression it means your sample size is too small or you have too many independent variables.
Most science experiments will have two independent variables. Fundamentally, an experiment will want as few variables as possible for better results.
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There are FAR TOO MANY possible variables that are unknown. That is a question best answered by the company's legal adviser or insuror.