Decision Rules and Guard Banding

Calibrating an item of test equipment or a measuring instrument is a way to manage the risk involved with using the item for critical measurements. When an item is calibrated, the measurement results are usually found either within or outside of its manufacturer’s accuracy specifications and the results are labeled as either ‘pass/fail’ or ‘in-tolerance/out-of-tolerance’. According to ISO/IEC 17025, these labels are considered to be ‘Statements of Conformity’ as the calibration laboratory is providing the customer with a general statement as to the accuracy of the measurement results.

The new 2017 revision of the ISO/IEC 17025 standard states that those statements of conformity must take into account a ‘Decision Rule’ that is to be applied when reporting the results to customers. ISO/IEC 17025:2017 defines the decision rule as a ‘rule that describes how measurement uncertainty is accounted for when stating conformity with a specified requirement’. The decision rule basically describes how a calibration laboratory uses the associated measurement uncertainty value to determine whether or not a measurement result passes or fails the accuracy requirements.

Guard Banding

Guard banding is how the decision rule is applied to the measurement result. Guard banding establishes the new specifications that determine whether or not a measurement is in-tolerance or out-of-tolerance. Guard banding is a safety factor built into the measurement process that is usually more restrictive than the manufacturer’s accuracy specifications of the instrument and can reduce the probability of making an incorrect conformance decision. This reduces the risk associated with making the measurement and increases confidence in the measurement result.

Customer Communication

The new revision of the ISO/IEC 17025 standard requires the decision rule to be defined, documented and agreed to by both the customer and the calibration laboratory. It is required that this communication is incorporated into the contract review process to show that it is documented and acceptable to all parties. The decision rule must also be stated in the calibration certificate and reported as part of the calibration results.

It doesn’t matter which party defines the decision rule. The calibration laboratory will likely offer a standard decision rule that they apply when evaluating the measured values and may also provide the customer with different variations of guard bands for the customer to choose from. The customer may also have an established decision rule that they instruct the calibration laboratory to use.

The most important part of the communication of the decision rule is that the customer clearly understands the decision rule and the risk associated with accepting the rule. Unfortunately, many customers don’t know what a decision rule is and don’t completely understand measurement uncertainty so they cannot properly evaluate the risk associated with their statements of conformity. Many customers just accept the decision rule provided by the laboratory and expect that it is adequate for their needs.

Customers must understand the requirements to choose the decision rule and risk that is appropriate for their organizational requirements and applications. For example, the decision rule for equipment used on a production floor might be different from the equipment used in their quality laboratory. Calibration laboratories should provide the education and guidance necessary to allow the customer to make the proper decision.

ILAC document, ILAC G8:09/2019 – ‘Guidelines on Decision Rules and Statements of Conformity’ is a reference document available to assist calibration laboratories in the use of decision rules when issuing statements of conformity and also can provide customers with the information they need when considering decision rules for their organization.

Decision Rules

To make statements of conformity and to report decision rules, calibration laboratories have several options that they can communicate to the customers. Different guard banding methods all have various levels of risk of reporting an incorrect measurement decision. The most common options are outlined further below.

ISO/IEC 17025 specifically states that decision rules are required when statements of conformity are made. If statements of conformity are not provided to the customer, then a decision rule is not needed and the information does not need to be included on the calibration certificate.

Many calibration laboratories will provide the customer with only the measurement values and the associated measurement uncertainty so that the customer can make their own informed decision whether or not the values meet their needs.

If the measurement uncertainty values are not used in the decision rule, then the pass/fail criteria will be the manufacturer’s specifications. This is called ‘simple acceptance’ and is usually the default method chosen. Depending on the measurement value and the expanded measurement uncertainty for the measurement, there could be a probability of a false accept, where the measurement is reported in-tolerance, but is actually out-of-tolerance.

There are numerous methods for using measurement uncertainty when used in a decision rule. The most common is where the guard band equals the expanded measurement uncertainty value and the acceptance limit is determined by reducing the manufacturer’s specification by the value of the expanded measurement uncertainty. While this minimizes the probability of a false accept, it increases the probability of false reject which may also have undesirable consequences for the customer.

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