The primary motivation for guardbanding is to control the risk of accepting an out-of-tolerance unit or rejecting an in-tolerance unit.
The ISO 17025 standard requires that measurement uncertainty be taken into account when statements of compliance are made. In other words, when a calibration laboratory calibrates an instrument and produces a calibration certificate indicating that the instrument “passed” or “failed”, it is important that, for each test point, the measurement uncertainty be first calculated and then used in the determination of the test result.
The ISO / IEC 17025 standard requires that the guardbanding technique be applied for each test point when the test uncertainty ratio (TUR) is less than 4: 1. The technique of guardbanding can lead to a situation where it is not possible to determine, within the confidence interval, the statement of compliance or non-compliance for the device under test. In this case, the declaration of conformity is “undetermined.”
Declaration of conformity for TUR* lower than 4: 1
• Ustd : Uncertainty of the standard (% of reading)
• Uinstrument : Uncertainty of the measurement system (% of reading)
• Resolution equipment under test : Resolution of the equipment under test (% of reading)
Zone of compliance calculated according to a new tolerance
(*) Le TUR se définit comme suit :
• TUR : Test Uncertainty Ratio
• Tolerance : Tolerance of the equipment under test
• Ustd : Uncertainty of the standard
• Uinstrument : Uncertainty of the measurement system
• Uresolution : Resolution of the equipment under test
ILAC G8 : Guidelines on Assessment and Reporting of Compliance with Specification
CLAS Requirements Document 3 (November 2009): Minimum Requirements for Measurement Standards for Laboratory Certification