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Measurement uncertainty in environmental analysis
 

Measurement uncertainty in environmental analysis

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    Measurement uncertainty in environmental analysis Measurement uncertainty in environmental analysis Presentation Transcript

    • Measurement Uncertainty in Environmental Analysis Todd R. Crawford, BA Center for Toxicology and Environmental Health, LLC
    • Standard Procedures
      • ISO 1993 – ‘Guide to the Expression of Uncertainty in Measurement’
      • NIST 1994 – ‘Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results’
    • Uncertainty Statements
      • Inform the reader of how sure the writer is that the answer lies within that range.
    • Uncertainty Statement?
      • “…if by any chance our value is shown to be in error by more than 1 part in 1,000, we are prepared to eat the apparatus and drink the ammonia.”
      • Attrib. Dr. C.H. Meyers
    • Error
      • Error is a single value – an idealized concept, error cannot be known exactly.
      • Error is the difference between the true value and the result of the measurement.
    • Uncertainty
      • Uncertainty is the state of knowledge about the unknown error.
      • Uncertainty is given as the range in which the error is to be expected.
    • Accuracy
      • Accuracy is the closeness of a measurement to the true value.
      • Accuracy is expressed as the percent recovery.
    • Precision
      • Precision is the closeness of agreement between repeated measurements.
      • Precision is expressed as the relative standard deviation (RSD).
    • Measurement Uncertainty in Environmental Analysis
      • The true value cannot be known.
      • The reported value is a ‘consensus’ value.
    • Reporting Uncertainty (ISO)
      • Reporting is required when:
        • The information is relevant to the application of the results,
        • When the client requires it,
        • When the Uncertainty affects compliance with a specification limit.
    • Evaluating Uncertainty (ISO)
      • Evaluating is required for calibrations.
      • Procedures for estimating uncertainty are needed and need to be applied.
    • Data Quality
      • The Uncertainty of a result is a quantitative indication of its quality.
      • Quantitative Uncertainty allows comparison of the results.
      • Measurement Uncertainty needs to be taken into account when interpreting the data.
    • TPH in Soil
      • Bioremediation of a former petroleum terminal to residential clean-up standards.
      • Approximately 500,000 cy of soil treated.
      • Approximately 15,000 samples analyzed on-site over five years.
      • On-site data demonstrated compliance with the remedial criteria.
    • Interlaboratory Study
      • Measure the Accuracy and Precision of Five Labs Analyzing TPH in Soil by GC-FID.
      • Homogenous soil containing biodegraded diesel fuel with 10% moisture.
      • Soil submitted in blind triplicates to each lab.
      • Each lab is certified for this analysis.
    • Summary Results (n=15)
      • Control Lab
        • TPH = 921 ppm
        • RSD = 2.4%
      • Independent Labs
        • TPH = 755 ppm
        • RSD = 35%
    • Assessing Measurement Uncertainty
      • Any set of numbers (n>3) has a mean and standard deviation.
      • Complex statistical tests are beyond the expertise of most data users.
      • Most data sets are too small to assess with any confidence.
    • Charting to Evaluate Uncertainty
      • Rank the data in order of concentration.
      • Normalize the data against the maximum value.
      • Chart the value against its rank, where rank equals 1/n.
      • Determine the slope and correlation coefficient.
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    • Conclusion
      • Forensic environmental investigations require evaluation of measurement uncertainty.
      • Quantitative results are most useful when reported in the context of their measurement uncertainty.
    • Conclusion
      • Measurement uncertainty is significantly improved with the familiarity of the laboratory to the analysis.
        • On-site is better than Off-site!
      • Measurement uncertainty can be evaluated (more effectively) by charting.
    • Acknowledgements
      • John E. Ross, de maximis, Inc. (Charter International Oil Company)
      • James S. Smith, Les Eng, Trillium, Inc.
      • Willem Schre ü der, Principia Mathematica, Inc.
      • Glenn Millner, Center for Toxicology and Environmental Health, LLC