ASTM_D_7729_-_12.pdf

Designation:D772912Standard Practice forDetermining and Expressing Precision of MeasurementResults,in the Analysis of Water,as Relative StandardDeviation,Utilizing DQCALC Software1This standard is issued under the fixed designation D7729;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon()indicates an editorial change since the last revision or reapproval.1.Scope1.1 This Practice describes a procedure for developing agraphical model of relative standard deviation vs concentrationfor a analytical methods used in the analysis of water(methodsthat are subject to non-additive random errors)for the purposeof assigning a statement of noise or randomness to analyticalresults(commonly referred to as a precision statement),ineither a manual or an automated fashion.1.2 Data analysis and modeling is done with D19 AdjunctDQCALC(an Excel based tool).1.3 The values stated in SI units are to be regarded as thestandard.The values given in parentheses are for informationonly.1.4 This standard does not purport to address all of thesafety concerns,if any,associated with its use.It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2.Introduction2.1 An understanding of the uncertainty associated withmeasurement results is necessary for evaluating the utility ofthose results.Without a reported uncertainty estimate,users ofmeasurement results are unable to determine if the data aresufficiently precise for any specific data use.2.2 Measurement Uncertainty is most generally understoodto be“a parameter characterizing the dispersion of the quantityvalues being attributed to a measurand”(from InternationalVocabulary of Metrology(VIM)2.26).This definition can beimplemented as an expression(“uncertainty statement”)asso-ciated with an reported measurement that represents thestatistically based(TypeAestimate)dispersion of experimentalresults around a reported value.2.3 There is no universally agreed upon format or nomen-clature for uncertainty statements.The literature offers sugges-tions ranging from simple expressions of standard deviation or“fractional uncertainty”(standard deviation divided by re-ported result)to confidence intervals to detailed“uncertaintyreports”.2.4 In addition to the“random”errors encompassed in theideas expressed in 1.1 and 1.2,above,there are also“system-atic”errors,biases,that can be considered as part of uncer-tainty.The literature is not consistent on how unknown bias isconsidered in an uncertainty statement.For purposes of thisStandard,bias is assumed to have been corrected for orinsignificant in the reported results,and bias is not specificallyincorporated in the proposed uncertainty statement.2.5 For purposes of this Standard,the terms“MU”,uncer-tainty statement,or measurement uncertainty will be usedsynonymously to designate the expression accompanying mea-surement results for the purpose of assessing the utility of thoseresults.2.6 This Standard proposes the use of fractional uncertaintyor Relative Standard Deviation(RSD)as the expression ofMU.2.7 Traditionally,in the generation and publication of datarelated to the analysis of water,a continuous function(model)describing the relationship of uncertainty(as standard devia-tion)to concentration is not available.To compensate for thislack,discrete points bounding certain levels of uncertainty arecalculated,for example,“detection limits”(typically around33%RSD)and“quantitation limits”(often around 10%RSD).Results are flagged to indicate their relationship to one of theselimits.Alternatively,this Practice directs the creation of amodel of uncertainty(RSD vs concentration)which allowsassignment of a discrete uncertainty estimate to any resultvalue measured within the range of modeled data.2.8 This Practice is based on the use of the DQCALCsoftware that was developed to simplify the calculation of theIQE Inter-laboratory Quantitation Estimate(D6512).ThisPractice is restricted to the development of an uncertaintymodel for the reporting of MU within a single laboratory.In1This practice is under the jurisdiction of ASTM Committee D19 on Water andis the direct responsibility of Subcommittee D19.02 on Quality Systems,Specification,and Statistics.Current edition approved April 1,2012.Published September 2012.DOI:10.1520/D772912.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 addition to providing an estimate of single-laboratory measure-ment uncertainty,the DQCALC software automatically calcu-lates LC from Curie,equivalent to EPAs MDL,and theASTM Detection Estimate for a single lab(this utilizes a“3sigma”tolerance interval rather than the standard confidenceinterval).2.9 This Practice provides the tools to allow