text: how are control variables and expermental variables used in scientific investigations?   data is collected for various values of the experimental variable while the controlable variables are kept constant.  in chemical and some biological laboratory tests where one wishes to quantify a particular element or compound in a sample, the procedure calls for using a known standard and/or a control.  sometimes there is a high contol and a low control, these being samples of carefully established known values.  where results are to be primarily qualitative, a positive control and a negative control are often used.