ASTM_D_7782_-_13.pdf

Designation:D778213Standard Practice forDetermination of the 99%/95%Critical Level(WCL)and aReliable Detection Estimate(WDE)Based on Within-laboratory Data1This standard is issued under the fixed designation D7782;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.NoteBalloted information was included and the year date changed on March 28,2013.1.Scope1.1 This practice provides a procedure for computing a99%95%Within-laboratory Detection Estimate(WDE)andthe associated critical level/value(WCL).The WDE is theminimum concentration,with false positives and false negativeappropriately controlled,such that values above these mini-mums are reliable detections.The WCL is the point at whichonly false positives are controlled appropriately.A false posi-tive is the reporting of an analyte as present when the analyteis not actually present;false negatives are reports of analyteabsence when the analyte is actually present.This practice isdistinguished from the Interlaboratory Detection Estimate(IDE)practice in that the IDE Standard utilizes data frommultiple,independent laboratories,while this practice is for useby a single laboratory.The IDE would be utilized whereinterlaboratory issues are of concern(for example,limits forpublished methods);this practice(and values derived from it)are applicable where the results from a single laboratory,singleoperator,single instrument,etc.are involved(for example,inunderstanding,censoring and reporting data).1.2 The establishment of a WDE involves determining theconcentration below which the precision and bias of ananalytical procedure indicates insufficient confidence in false-positive and false-negative control to assert detection of theanalyte in the future analysis of an unknown number ofsamples.Most traditional approaches attempt to determine thisdetection“limit”by estimating precision at only a single,arbitrary point.The WDE approach is intended to be a moretechnically rigorous replacement for other approaches forestimating detection limits.The WDE practice addresses anumber of critical issues that are ignored in other approaches.1.2.1 First,rather than making a single-point estimate ofprecision,the WDE protocol requires an estimate of precisionat multiple points in the analytical range,especially in therange of the expected detection limit.These estimates are thenused to create an appropriate model of the methods precision.This approach is a more credible way to determine the pointwhere relative precision has become too large for reliabledetection.This process requires more data than has beenhistorically required by single-point approaches or by pro-cesses for modeling the relationship between standard devia-tion and concentration.1.2.2 Second,unlike most other approaches,the WDEprocess accounts for analytical bias at the concentrations ofinterest.The relationship of true concentration to measuredconcentration(that is,the recovery curve)is established andutilized in converting from as-measured to true concentration.1.2.3 Third,most traditional approaches to detection limitsonly address the issue of false positives.Although falsenegatives may not be of concern in some data uses,there aremany uses where understanding and/or control of false nega-tives is important.Without the false-negative-controlinformation,data reported with just a critical-level value areincompletely described and the qualities of data at these levelsincompletely disclosed.1.2.4 Fourth and last,the WDE standard utilizes astatistical-tolerance interval in calculations,such that futuremeasurements may reasonably be expected to be encompassedby the WDE 90%of the time.Many older approaches haveused the statistical confidence interval,which is not intended toencompass individual future measurements,and has beenmisunderstood and misapplied.Procedures using the confi-dence interval cannot provide the stated control when thedetection-limit value is applied to future sample results;suchapplication is the primary use of these values.1.3 To summarize,the WDE is computed to be the lowesttrue concentration at which there is 90%confidence that asingle(future)measurement(from the studied laboratory)willhave a true detection probability of at least 95%and a truenon-detection probability of at least 99%(when measuring ablank sample).For the laboratory in the study,the critical value1This 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 March 28,2013.Published April 2013.DOI:10.1520/D778213.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 is the true concentration at which,on average,(