CAC GL 54-2004GUIDELINES ON MEASUREMENT UNCERTAINTY.pdf

CAC/GL54-2004
Page 1 of 2
GUIDELINES ON MEASUREMENT UNCERTAINT
CAC/GL54-2004
Introduction
It is important and required by TSO EC 17025: 999 that analysts arc aware of thc uncertainty associatcd
with cach analytical rcsult and cstimatcs that uncertainty. Thc mcasurcmcnt uncertainty may bc dcrivcd by a
number of procedures. Food analysis laboratories are required, for Codex purposes, to be in control, use
collaboratively tested or validated methods when available, and verify their application before taking them
into routine use. Such laboratories therefore have available to them a range of analytical data which can be
used to estimate their measurement uncertainty
These guidelines only apply to quantitative analysis
ntitative analytical results take the form of"a +2u or a t U" is the best estimate of th
true value of the concentration of the measurand(the analytical result)and"u is the standard uncertainty
nd U(equal to 2u )is the expanded uncertainty. The range"a= 2u represents a 95% level of confidence
where the true value would be found. The value of
or 2u is the value which is normally used and
rcportcd by analysts and is hcrcaftcr referred to as"mcasurcmcnt uncertainty " and may be cstimatcd in a
numbcr of diffcrcnt ways
Terminology
The international definition for Measurement Uncertainty is
Parameter, associated with the result of a measurement, that characterises the dispersion of the values that
could reasonably be attributed to the measurand'"
NOTES
The parameter may be, for example, a standard deviation (or a given multiple of id), or the half-width
of an interval having a statcd Icvcl of confidcncc
Uncertainty of measurement comprises, in general, many components. Some of these components
may be evaluated from thc statistical distribution of results of a scrics of mcasurcmcnts and can bc
charactcriscd by cxpcrimcntal standard deviations
c othcr componcnts, which can also be
characterised by standard deviations, are evaluated from assumed probability distributions based on
experience or other information
It is undcrstood that the result of a mcasurcmcnt is thc best cstiumatc of thc valuc of a measurand. and
that all componcnts of uncertainty, including thosc arising from systematic cffccts, such as componcnts
associated with corrections and rcfcrcncc standards, contributc to thc dispersion
Recommendations
The measurement uncertainty associated with all analytical results is to be estimated
he measurement uncertainty of an analytical result may be estimated by a number of procedures
notably those described by ISO(1 )and EURACHEM(2). These documents recommend procedures
based on a component-by-component approach, method validation dala, internal quality control dala
and pro iciency test data. The need to undertake an estimation of the measurement uncertainty using
the ISO componcnt-by-componcnt approach is not ncccssary if the othcr forms of data arc availablc
and uscd to estimate thc uncertainty. In many cascs the ovcrall uncertainty may be dctcrmincd by an
As outlined in Codex GL 27-1997guidclincs for the Asscssmcnt of the Compctcncc of Tcsting Laboratorics
Involved in the Import and Export of Food
International vocabulary of basic and general terms in metrology, ISO 1993, 2 Dition