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Measurement Uncertainty in Environmental Analysis

Todd R. Crawford, BA

Center for Toxicology and

Environmental Health, LLC

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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’

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Uncertainty Statements

Inform the reader of how sure the writer is that the answer lies within that range.

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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

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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.

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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.

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Accuracy

• Accuracy is the closeness of a measurement to the true value.

• Accuracy is expressed as the percent recovery.

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Precision

• Precision is the closeness of agreement between repeated measurements.

• Precision is expressed as the relative standard deviation (RSD).

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Measurement Uncertainty in Environmental Analysis

• The true value cannot be known.

• The reported value is a ‘consensus’ value.

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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.

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Evaluating Uncertainty (ISO)

• Evaluating is required for calibrations.

• Procedures for estimating uncertainty are needed and need to be applied.

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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.

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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.

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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.

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Summary Results (n=15)

• Control Lab– TPH = 921 ppm

– RSD = 2.4%

• Independent Labs– TPH = 755 ppm

– RSD = 35%

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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.

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Charting to Evaluate Uncertainty

1. Rank the data in order of concentration.

2. Normalize the data against the maximum value.

3. Chart the value against its rank, where rank equals 1/n.

4. Determine the slope and correlation coefficient.

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Summary of MA-EPH Round Robin 2 Soil Data

0

6000

12000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

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Normalized EPH Data versus Lab Rank

y = 1.0374x + 0.464

R2 = 0.72730.00

0.50

1.00

1.50

2.00

2.50

0.00 0.20 0.40 0.60 0.80 1.00 1.20

Lab Rank (n=30)

Norm

alize

d Con

cent

ration

(mea

n = 37

29)

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Normalized EPH Data without Outliers

y = 0.666x + 0.6183

R2 = 0.9601

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

0.00 0.20 0.40 0.60 0.80 1.00 1.20

Lab Rank (n=30)

Norm

alize

d Con

cent

ration

(mea

n = 37

29)

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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.

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Conclusion

• Measurement uncertainty is significantly improved with the familiarity of the laboratory to the analysis.

– On-site is better than Off-site!On-site is better than Off-site!

• Measurement uncertainty can be evaluated (more effectively) by charting.

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Acknowledgements

• John E. Ross, de maximis, Inc. (Charter International Oil Company)

• James S. Smith, Les Eng, Trillium, Inc.

• Willem Schreüder, Principia Mathematica, Inc.

• Glenn Millner, Center for Toxicology and Environmental Health, LLC