The international standard ISO/IEC 17025:2017 states that when a statement of conformity is used by a calibration laboratory to indicate whether a measurement value ‘passes’ or ‘fails’ based on the defined specifications, a decision rule needs to be applied.
Most decision rules currently used in calibration laboratories use the calculated measurement uncertainty value that takes into account all of the possible influences within the measurement system that can impact the measurement result.
Guardbanding is the technique that is used to implement the decision rule. It is applied as an balanced offset that is placed around the specification limit with a width equal to some portion of the measurement uncertainty value. The most common guardband value is the expanded uncertainty calculated at a 95 % confidence level, although other percentage values can be used depending on the risks that can be accepted for the measurement.
The guardband establishes an upper and lower acceptance limit and is used to determine whether the measurement result either passes or fails in relation to the specification. Different guardbanding values can also be used to determine equipment adjustment criteria or used for other statistical analysis.
Guardbanding is a way to quantify and mitigate the risks associated with a measurement when making a statement of conformity. The main reason for using guardbanding is to minimize the risk of declaring that a measurement value passes when it actually fails, called a ‘false accept’, or declaring that a measurement value fails when it actually passes, called a ‘false reject’.
A standard way of looking at the risks associated with a measurement is by using the terms, “Consumer’s Risk” and “Producer’s Risk.”
Consumer’s Risk describes the ‘false accept’ concept where a product that does not meet the manufacturer’s specifications or other quality requirements is indicated as ‘passed’ during the inspection or measurement process and is sent to the customer. This can result in product recalls or poor levels of customer satisfaction.
Producer’s Risk describes the ‘false reject’ concept where products that would normally meet the manufacturer’s specifications or quality requirements are characterized as ‘failed’ during the inspection or measurement process and rejected from the production line. This can result in unnecessary costs and manpower as the products are either scrapped or reworked to meet the specifications.
The use of guardbanding does not completely eliminate the probability of the risk of a false accept or false reject condition, but can assist in controlling the measurement outcome.
A simple example of using the guardband technique is described below.
In this example, the nominal value for the measurement is 10 volts, the specification is ± 1 volt, and the expanded measurement uncertainty value at the 95% confidence level is 0.2 volt.
The specification zone for the measurement will be between 9 volts and 11 volts.
The acceptance zone for the measurement will be between 9.2 volts and 10.8 volts. The acceptance zone is determined by reducing the specification zone by the measurement uncertainty value from both the high specification limit and the low specification limit.
If the measurement value falls within the acceptance zone, such as 10.3 volts, the measurement can be stated as ‘Pass’. If the measurement value falls outside of the acceptance zone, such as 11.3 volts, the measurement would be stated as ‘Fail’.
Some calibration laboratories may use a designation of ‘undetermined’ for measurement values that fall within the guardband zone, such as 10.9 volts, as it is technically not possible to determine the compliance or non-compliance for the measurements in this area within their stated measurement uncertainty.
One of the key aspects of the use of guardbanding is that the method used for the guardbanding is clearly defined and communicated to the customer. Some customers may determine that guardbanding is not required in the reporting of their measurements. In this case the guardband would equal zero, so that the acceptance zone is equal to the manufacturer’s specification zone.
Every customer of calibration services needs to determine the acceptable specification limits required based on their operational requirements and evaluate the published calibration ranges and measurement uncertainties listed in the Scope of Accreditation of their calibration supplier to ensure that they are adequate for each piece of measurement and test equipment.
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