This is the calibration technique of choice in almost any method of analysis, chromatography included. A series of standards with known concentrations of the analyte is analyzed under the same conditions as the sample to establish the calibration curve as the least square fit through the data points. Once linearity is verified, the two parameters, d and S, are determined and used subsequently to convert the responses in test samples to analyte concentrations. The obvious advantage of the external standard technique is that a single calibration can serve a multitude of samples. Periodic analysis of a well preserved (test)sample suffices to verify stability of the response or to signal the need for recalibration. In the case of a linear calibration curve recalibration can be a relatively simple single-point recalibration. Hence, the external standard technique is particular popular in repetitive, routine analysis of many similar samples. It gains even greater weight when several calibration curves can be used to determine a number of components in each test sample. For a more extensive discussion of calibration curves the reader is referred to the article on Linear Calibration Curve.