A calibration curve is developed by analyzing several standards of known concentration and measuring the test method's relative response at each concentration. In this way, the actual concentration of samples that fall anywhere within this range can be accurately estimated. Two or three standards and a blank or zero solution typically are used to create a calibration curve for most colorimetric indicator kits. It is recommended to use standards that cover the applicable action levels, and the expected range of concentrations present at the site if possible.