Calibration: A procedure which enables a series of simply obtainable but inaccurate measurements of some quantity of interest to be used to provide more precise estimates of the required values. Assume, for instance, there is a well-established, accurate process of measuring the concentration of a given chemical compound, but that it is too expensive and cumbersome for routine use. A cheap and simple to apply an alternative is developed that is, though, known to be imprecise and possibly subject to bias. By using both methods or ways over a range of concentrations of compound, and applying regression analysis to the values from the cheap way and the corresponding values from accurate method, a calibration curve can be constructed which may, in future applications, be used to read off the estimates of the needed concentration from the values given by less involved, inaccurate procedure.