The operator prepares a series of standards across a range of concentrations near the expected concentration of analyte in the unknown. The concentrations of the standards must lie within the working range of the technique (instrumentation) they are using. Analyzing each of these standards using the chosen technique will produce a series of measurements. For most analyses a plot of instrument response vs. concentration will show a linear relationship. The operator can measure the response of the unknown and, using the calibration curve, can interpolate to find the concentration of analyte.
The data - the concentrations of the analyte and the instrument response for each standard - can be fit to a straight line, using linear regression analysis.  This yields a model described by the equation y = mx + y, where y is the instrument response, m represents the sensitivity, and y is a constant that describes the background.  The analyte concentration (x) of unknown samples may be calculated from this equation.
Many different variables can be used as the analytical signal.  For instance, chromium (III) might be measured using a chemiluminescence method, in an instrument that contains a photomultiplier tube (PMT) as the detector.  The detector converts the light produced by the sample into a voltage, which increases with intensity of light. The amount of light measured is the analytical signal.