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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?
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:
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:
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
ASME B126.96.36.199:2001 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?
L-A-B Policies Page – Click Here
The GUM: JCGM100:2008 Evaluation of measurement data – Guide to the expression of uncertainty of measurement