The control variable (or scientific constant) in scientific experimentation is the experimental element which is constant and unchanged throughout the course of the investigation. The control variable strongly influences experimental results, and it is held constant during the experiment in order to test the relative relationship of the dependent and independent variables. The control variable itself is not of primary interest to the experimenter.
A variable in an experiment which is held constant in order to assess the relationship between two other variables, is the control variable. A control variable is the one element that is not changed throughout an experiment, because its unchanging state allows the relationship between the other variables being tested to be better understood.
Essentially, a control variable is what is kept the same throughout the experiment, and it is not of primary concern in the experimental outcome. Any change in a control variable in an experiment would invalidate the correlation of dependent variables (DV) to the independent variable (IV), thus skewing the results.
In any system existing in a natural state, many variables may be interdependent, with each affecting the other. Scientific experiments tests the relationship of an IV –that element which is manipulated by the experimenter– to the DV –that element affected by the manipulation of the IV. Any additional independent variable can be a control variable.
For example, in the experimental verification of Boyle's law (P = T * V), where Pressure, Temperature, and Volume are all variables, to test the resultant changes to any of these variables requires at least one to be kept constant. This is in order to see comparable experimental results in the remaining variables. If we choose to make Pressure the control variable and don't allow it to change throughout the course of the experiment, we can quickly establish the relationship between Temperature and Volume by changing the value for one or the other. For instance, if we raise the Temperature (making temperature the IV in this case) then the Volume must become lower (making it the DV in this case).
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