Techniques for Improving the Accuracy of Calibration in the Environmental Laboratory
Dennis A. Edgerley
Quanterra Environmental Services, 880 Riverside Parkway, West Sacramento, California 95605
Paper published in the Proceedings of WTQA '98
(14th Annual Waste Testing & Quality Assurance Symposium), pp. 181-187.
Consistent and reliable procedures for generating calibrations are essential to accurate laboratory results. Unfortunately the interpretation of acceptable practice is often based on misunderstanding or derived from practices commonly utilized in non-environmental methods, and therefore does not provide a reliable means for maintaining data quality. This paper presents a demonstration that some common practices used in the calculation and evaluation of calibration factors, including the use of unweighted regression and the associated correlation coefficient, are inappropriate for environmental analysis due to high relative errors which result at the low end of the curve. Alternate criteria for evaluation of calibration curves are proposed based on the Relative Standard Error (%RSE). Statistical derivations and examples are presented to demonstrate how this approach provides an improved measure for the evaluation of calibration data based on weighted regression. Other related considerations for assessing acceptability of calibration data are also presented.
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