Uncertainty

The definition of measurement uncertainty which has achieved global acceptance in the metrology community is contained in the International vocabulary of Metrology (VIM).
JCGM200:2012 International vocabulary of metrology – Basic and general concepts and associated terms

“non-negative parameter characterizing the dispersion of the quantity values being attributed to a measurand, based on the information used”

Knowledge of the uncertainty of measurement of testing and calibration results is fundamentally important for laboratories, their clients and all institutions using these results for comparative purposes. Competent laboratories know the performance of their methods and the uncertainty associated with the results. Uncertainty of measurement is a very important metric of the quality of a result or a testing method. Other such metrics are reproducibility, repeatability, robustness and selectivity.

What does Uncertainty of Measurement include?

• Measurement uncertainty includes components arising from systematic effects, such as components associated with corrections and the assigned quantity values of measurement standards, as well as the definitional uncertainty. Sometimes estimated systematic effects are not corrected for but, instead, associated measurement uncertainty components are incorporated.
• The parameter may be, for example, a standard deviation called standard measurement uncertainty (or a specified multiple of it), or the half-width of an interval, having a stated coverage probability.
• Measurement uncertainty comprises, in general, many components. Some of these may be evaluated by Type A evaluation of measurement uncertainty from the statistical distribution of the quantity values from series of measurements and can be characterized by standard deviations. The other components, which may be evaluated by Type B evaluation of measurement uncertainty, can also be characterized by standard deviations, evaluated from probability density functions based on experience or other information.
• In general, for a given set of information, it is understood that the measurement uncertainty is associated with a stated quantity value attributed to the measurand. A modification of this value results in a modification of the associated uncertainty.

What is to be considered in an uncertainty of measurement (uncertainty budget)?

Uncertainty components/budgets are a combination of many factors that may include, but are not limited to:

• Reference standards (inherited uncertainty, long-term stability, resolution or readability)
• Reference materials (inherited uncertainty, long-term stability)
• T/C methods used (process or method repeatability, reproducibility, stability)
• Ancillary Equipment used (inherited uncertainty, long-term stability, resolution or readability)
• Environmental conditions (those affecting the measurement process or equipment)
• Properties and condition of item being tested
• Operator (technicians’ skill, repeatability or reproducibility)
• Known physical characteristics of reference standards, equipment used, or items under test (bias and correction values, coefficients of thermal expansion, density)

Measurement Uncertainty Budget Calculator

Reporting the Uncertainty of Measurement

When reporting the measurement and its associated uncertainty, the certificate / report may include the contributors of uncertainty or a record within the quality management system shall be present for the identification of the contributors that affected the measurement taken. In many cases, reporting the expanded uncertainty in a certificate or report is more than likely to be expressed at a coverage level of k = 2 and a confidence level of approximately 95%. Reporting the uncertainty of measurement is also critical in maintaining traceability. Without the uncertainty reported on the certificate / report, the chain of traceability is broken and confidence cannot be maintained in the measurement taken.

How is the uncertainty of a measurement taken into account when making a statement of compliance with a specification of tolerance limit?

Where a property of interest, such as the error of indication of a measurement device is measured in order to decide whether or not the item conforms to a specified requirement, such a test of conformity is comprised of three operations:

• measure the property of interest;
• compare the measurement result with the specified requirement;
• decide on a subsequent action.

Typically, a laboratory has a previously established and stated decision rule which is applied to a measurement in order to determine conformance. Some methods prescribe the decision rule to be used.

Guidance is available regarding the formulation of a decision rule:

ILAC G8:2009 Guidelines on assessment and reporting of compliance with specification

• If the specification limits are not breached by the measurement result, extended by the expanded uncertainty interval at a level of confidence of 95%, then compliance with the specification can be stated.

ASME B89.7.3.1:2001 Guidelines for decision rules: Considering measurement uncertainty in determining conformance to specifications.

ISO 14253-1:2013 Geometrical Product Speculations GPS | Inspection by measurement of workpieces and measuring equipment | Part 1: Decision rules for proving conformance or non-conformance with specifications.

Handbook for the application of ANSI/NCSL Z540.3:2006 Requirements for the calibration of measuring and test equipment

Where do I find information on L-A-B Uncertainty Requirements and Policies?