Treatment of uncorrected measurement bias in uncertainty estimation for chemical measurements
- 396 Downloads
Consistent treatment of measurement bias, including the question of whether or not to correct for bias, is essential for the comparability of measurement results. The case for correcting for bias is discussed, and it is shown that instances in which bias is known or suspected, but in which a specific correction cannot be justified, are comparatively common. The ISO Guide to the Expression of Uncertainty in Measurement does not provide well for this situation. It is concluded that there is a need for guidance on handling cases of uncorrected bias. Several different published approaches to the treatment of uncorrected bias and its uncertainty are critically reviewed with regard to coverage probability and simplicity of execution. On the basis of current studies, and taking into account testing laboratory needs for a simple and consistent approach with a symmetric uncertainty interval, we conclude that for most cases with large degrees of freedom, linear addition of a bias term adjusted for exact coverage (“Ue”) as described by Synek is to be preferred. This approach does, however, become more complex if degrees of freedom are low. For modest bias and low degrees of freedom, summation of bias, bias uncertainty and observed value uncertainty in quadrature (“RSSu”) provides a similar interval and is simpler to adapt to reduced degrees of freedom, at the cost of a more restricted range of application if accurate coverage is desired.
KeywordsBias Uncertainty Recovery Uncorrected bias
The authors thank Marina Patriarca, ISS, Italy and Ivo Leito, Tartu University, Estonia and Elvar Theodorsson, University Hospital of Linköping, Sweden for stimulating and valuable discussions and contributions.
- 1.Hund E, Luc Massart D, Smeyers-Verbeke J (2001) TRAC 20(8):394–406Google Scholar
- 3.Hasselbarth W (2004) Accred Qual Assur 9:509–514Google Scholar
- 5.BIPM, IEC, IFCC, ILAC, ISO, IUPAC, IUPAP, OIML: Guide to the expression of uncertainty in measurement (1993) ISO, GenevaGoogle Scholar
- 7.ISO Guide 99 International vocabulary of metrology-basic and general concepts and associated terms VIM 3rd edn (2007) ISO, GenevaGoogle Scholar
- 13.Magnusson B, Naykki T, Hovind H, Krysell M (2003) Handbook for calculation of measurement uncertainty. NORDTEST report TR 537 Internet version 2003. Can be obtained from http://www.nordicinnovation.net/nordtest.cfm under link Rapporter. Cited June 2007
- 14.EN 10315 (2006) Routine method for analysis of high alloy steel by X-ray fluorescence spectrometry (XRF) by using a near by techniqueGoogle Scholar
- 16.Commission Regulations (EC) 333–2007, Laying down the method of sampling and analysis for the official control of the levels of lead, cadmium, mercury, inorganic tin, 3-MPCD, benzo(a)pyrene in food stuffGoogle Scholar
- 20.Phillips SD, Eberhardt KR, Parry B (1997) J Res Nat Inst Stand Technol 102:577–585Google Scholar
- 21.Franson MAH (1989) American Public Health Association. Standard methods for examination of water and wastewater, Washington DCGoogle Scholar
- 22.Eurachem/Citac guide (2000) Quantifying uncertainty in analytical measurement, 2nd edn. http://www.eurachem.org (Cited June 2007)