International Organization for Standardization (ISO)
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ISO/TS 21749:2005
Scope
This Technical Specification follows the approach taken in the Guide to the expression of the uncertainty of
measurement (GUM) and establishes the basic structure for stating and combining components of
uncertainty. To this basic structure, it adds a statistical framework using the analysis of variance (ANOVA) for
estimating individual components, particularly those classified as Type A evaluations of uncertainty, i.e. based
on the use of statistical methods. A short description of Type B evaluations of uncertainty (non-statistical) is
included for completeness.
This Technical Specification covers experimental situations where the components of uncertainty can be
estimated from statistical analysis of repeated measurements, instruments, test items or check standards.
It provides methods for obtaining uncertainties from single-, two- and three-level nested designs only. More
complicated experimental situations where, for example, there is interaction between operator effects and
instrument effects or a cross effect, are not covered.
This Technical Specification is not applicable to measurements that cannot be replicated, such as destructive
measurements or measurements on dynamically varying systems (such as fluid flow, electronic currents or
telecommunications systems). It is not particularly directed to the certification of reference materials
(particularly chemical substances) and to calibrations where artefacts are compared using a scheme known
as a “weighing design”. For certification of reference materials, see ISO Guide 35[14].
When results from interlaboratory studies can be used, techniques are presented in the companion guide
ISO/TS 21748 [15]. The main difference between ISO/TS 21748 and this Technical Specification is that the
ISO/TS 21748 is concerned with reproducibility data (with the inevitable repeatability effects), whereas this
Technical Specification concentrates on repeatability data and the use of the analysis of variance for its
treatment.
This Technical Specification is applicable to a wide variety of measurements, for example, lengths, angles,
voltages, resistances, masses and densities.
ISO/TS 21749:2005 history
2005ISO/TS 21749:2005 Measurement and uncertainty for metrological applications - Repeated measurements and nested experiments