Checking / Continual Validation (ASTM D3764) / On-Going Validation / Routine Validation / Verification. The assurance process by which the bias and precision performance determined during initial validation are shown to be maintained. The presentation of a reference sample to an analyser in order to determine the difference between the analyser result and the accepted reference value. A routine procedure to monitor the performance of the analyser over an extended period using control charts. No adjustment to the calibration of the analyser is made unless the control charts indicate a real change has occurred.