If an experiment has two independent variables how do you know which one effects the dependent variable?

If an experiment has two or more independent variable, how do you know which one effects the dependent variable?If an experiment has two independent variables how do you know which one effects the dependent variable?
You don't. When you do an experiment you have to set controls.



i.e. For one experiment hold indie variable 1 constant, and change indie variable 2. For another, hold variable 2 constant, and change 1. This should let you know if only one of the variables is affecting your dependent variable.