The set of actions used during set-up and periodic validation of the performance of an instrument or measurement system, specifically to establish a correlation between instrument output and final (reportable) results. Through calibration, the systematic error can be minimized and the accuracy of an instrument or measurement system established. Calibration is normally performed for an instrument on reference materials so that the instrument can be properly tuned using well characterized material. The result of the calibration(s) may be recorded in a document, called a calibration certificate, and is sometimes expressed as a calibration factor or a set of calibration factors, for example, in the form of a calibration curve. The calibration process should include an estimation of the associated uncertainty.