ASTM D7729-12
(Practice)Standard Practice for Determining and Expressing Precision of Measurement Results, in the Analysis of Water, as Relative Standard Deviation, Utilizing DQCALC Software
Standard Practice for Determining and Expressing Precision of Measurement Results, in the Analysis of Water, as Relative Standard Deviation, Utilizing DQCALC Software
SCOPE
1.1 This Practice describes a procedure for developing a graphical model of relative standard deviation vs concentration for a analytical methods used in the analysis of water (methods that are subject to non-additive random errors) for the purpose of assigning a statement of noise or randomness to analytical results (commonly referred to as a precision statement), in either a manual or an automated fashion.
1.2 Data analysis and modeling is done with D19 Adjunct DQCALC (an Excel based tool).
1.3 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: D7729 − 12
Standard Practice for
Determining and Expressing Precision of Measurement
Results, in the Analysis of Water, as Relative Standard
1
Deviation, Utilizing DQCALC Software
This standard is issued under the fixed designation D7729; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2.3 There is no universally agreed upon format or nomen-
clature for uncertainty statements. The literature offers sugges-
1.1 This Practice describes a procedure for developing a
tions ranging from simple expressions of standard deviation or
graphical model of relative standard deviation vs concentration
“fractional uncertainty” (standard deviation divided by re-
for a analytical methods used in the analysis of water (methods
ported result) to confidence intervals to detailed “uncertainty
that are subject to non-additive random errors) for the purpose
reports”.
of assigning a statement of noise or randomness to analytical
results (commonly referred to as a precision statement), in
2.4 In addition to the “random” errors encompassed in the
either a manual or an automated fashion.
ideas expressed in 1.1 and 1.2, above, there are also “system-
atic” errors, biases, that can be considered as part of uncer-
1.2 Data analysis and modeling is done with D19 Adjunct
tainty. The literature is not consistent on how unknown bias is
DQCALC (an Excel based tool).
considered in an uncertainty statement. For purposes of this
1.3 The values stated in SI units are to be regarded as the
Standard, bias is assumed to have been corrected for or
standard. The values given in parentheses are for information
insignificant in the reported results, and bias is not specifically
only.
incorporated in the proposed uncertainty statement.
1.4 This standard does not purport to address all of the
2.5 For purposes of this Standard, the terms “MU”, uncer-
safety concerns, if any, associated with its use. It is the
tainty statement, or measurement uncertainty will be used
responsibility of the user of this standard to establish appro-
synonymously to designate the expression accompanying mea-
priate safety and health practices and determine the applica-
surementresultsforthepurposeofassessingtheutilityofthose
bility of regulatory limitations prior to use.
results.
2. Introduction
2.6 This Standard proposes the use of fractional uncertainty
or Relative Standard Deviation (RSD) as the expression of
2.1 An understanding of the uncertainty associated with
MU.
measurement results is necessary for evaluating the utility of
those results. Without a reported uncertainty estimate, users of
2.7 Traditionally, in the generation and publication of data
measurement results are unable to determine if the data are
related to the analysis of water, a continuous function (model)
sufficiently precise for any specific data use.
describing the relationship of uncertainty (as standard devia-
2.2 Measurement Uncertainty is most generally understood tion) to concentration is not available. To compensate for this
to be “a parameter characterizing the dispersion of the quantity lack, discrete points bounding certain levels of uncertainty are
values being attributed to a measurand” (from International calculated, for example, “detection limits” (typically around
Vocabulary of Metrology (VIM) 2.26). This definition can be 33% RSD) and “quantitation limits” (often around 10% RSD).
implemented as an expression (“uncertainty statement”) asso- Results are flagged to indicate their relationship to one of these
ciated with an reported measurement that represents the limits. Alternatively, this Practice directs the creation of a
statisticallybased(TypeAestimate)dispersionofexperimental model of uncertainty (RSD vs concentration) which allows
results around a reported value. assignment of a discrete uncertainty estimate to any result
value measured within the range of modeled data.
2.8 This Practice is based on the use of the DQCALC
1
This practice is under the jurisdiction of ASTM Committee D19 on Water and
software that was developed to simplify the calculation of the
is the direct responsibility of Subcommittee D19.02 on Quality Systems,
IQE – Inter-laboratory Quantitation Estimate (D6512). This
Specification, and Statistics.
Practice is restricted to the development of an uncertainty
Current edition approved April 1, 2012. Published September 2012. DOI:
10.1520/D7729–12. model for the reporting of MU within a single laboratory. In
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