text: Data is collected for various values of the experimental variable while the controlable variables are kept constant.\n\nIn 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.\n\nWhere results are to be primarily qualitative, a positive control and a negative control are often used.