Specification Compliance Decisions with respect to "ISO17025 Calibration"

An uncertainty is a figure of merit associated with the actual measured value; the boundary limits within which the ‘true’ value lies. Contributors to this “potential for inaccuracy” include the performance of the equipment used to make the measurement, the test process or technique itself and environmental effects.

Additional imprecision may result from behaviour of the phenomenon or item being measured. A skilled metrologist will assess and combine these various components in an uncertainty budget. To prove that a product complies with specification (or doesn’t), the uncertainty must be less than the unknown’s specification.

In the following example, a measurement is performed on five different items: A, B, C, D and E. The results of these measurements are shown in the illustration below.

Without taking measurement uncertainty into account we could state that:

1. Devices A and B are within specification.
2. Device C is borderline.
3. Devices D and E are exceed specification.

However, when taking uncertainty into account:

1. We can say with high-confidence that device A is within specification (that is, even if the uncertainty is added to the measured value, the tolerance is not exceeded).
2. Devices B, C and D could be either in or out-of-specification (that is, the status of the product's compliance is "indeterminate").
3. Device E is out-of-specification.

Agilent’s Position on ‘Indeterminate’ Points

In the real world, the ‘indeterminate’ situation is largely impractical. Agilent has considered this situation at great length, in conjunction with input from its customers. The result is an approach that strikes a good balance between what is metrologically precise and what customers really want and so creating a realistic, commercially acceptable service.

Consequently, for Agilent's "ISO17025 Calibration" service, the assessment of A, B, C, D and E will be:

A = PASS
Performance is better than or equal to product specification. It is labelled PASS and a statement of conformity is issued.

B, C = PASS #
Performance falls within the measurement uncertainty guard band on the ‘in-specification’ side. The customer is notified on the measurement report that:

”All measured values indicated by this data report lie within specification. Due to measurement uncertainty, the true values of one or more of these points may lie outside of the specification, but there is a high probability that these values lie within specification.”

A certificate of calibration stating that the instrument meets specifications is issued. B and C are not adjusted to be closer to center.

D, E = FAIL
Performance falls within measurement uncertainty guard band on the ‘out-of-specification’ side and is considered a FAIL. A statement of conformity is issued only if D and E are adjusted to be in specification.