Experimental variables can fall under three different classifications, independent variables, dependent variables, and constant (or control) variables. An independent variable is one of the input variables. In other words, it is the measurement being tested. A dependent variable is one of the output variables. In other words, it is the effect being measured. Control variables are variables other than the ones being directly tested. They may have an effect on the experiment but since they are not being tested they are held constant to minimize affecting the experiment. In the case of titrating a solution of HCl against a solution of NaOH, the independent variable is the concentration of the NaOH solution that is being added (the input factor). The dependent variable is the concentration of the HCl solution (the variable being measured). A constant variable would be the chemical indicator being used (like phenolphthalein). Since the effect of this is not being measured, it should be kept the same throughout the experiment.