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Measurement Uncertainty

Measurement Uncertainty. Measurements. Accuracy - hitting the center of the target Precision - tight pattern of hits Bias - all two inches high Uncertainty - circle containing 95% hits. Traceability. Accuracy - close to system limitation Precision - range

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Measurement Uncertainty

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  1. Measurement Uncertainty

  2. Measurements • Accuracy - hitting the center of the target • Precision - tight pattern of hits • Bias - all two inches high • Uncertainty - circle containing 95% hits

  3. Traceability • Accuracy - close to system limitation • Precision - range • Bias - identified and corrected for • Acceptable Uncertainty - 2 sigma • Unbroken links to national standards

  4. Calibration Traceability • Unbroken linkage to national standards • Uncertainty - elements identified and quantified • Strong QC/QA program • Periodic MQA testing

  5. Measurement Traceability • Starts with calibration uncertainty • Identifies and quantifies all factors that affect the quality of a measurement • Requires reproducibility • Testing usually required to validate values

  6. How this affects you • Did I make the measurement right? • Is it reproducible? • What uncertainty is involved? • Can I cite them for a violation?

  7. 2 cases, more than 2 results • Package survey of radioactive material • Mammography machine measurements

  8. Package • 31x31x27 cm • Bicron RSO-50E • Calibrated by secondary laboratory • Maximum surface dose rate measured is 0.4 uSv/h (0.4 mR/h) • Traveling by air

  9. IATA (ST-1 Requirements) • Accuracy - so that correctly labeled • Precision/uncertainty - based on package surface to detector center distance • Bias - as high as a factor of 4 low

  10. ST-2 Measurement Guidance Correction factors for package and detector sizes

  11. Mammography Measurements • Exposure at skin entrance • Half value layer • Non-invasive kVp measurements • Mean glandular dose calculation

  12. X-ray vs. Mammo Probes

  13. Phantom Variability

  14. Temperature Effects

  15. HVL Measurements

  16. Non Invasive kVp Measurements

  17. Component Estimated Std. Dev. Meter Calibration 0.015 Temp/Pressure 0.01 Rate Dependence 0.01 Energy Dependence 0.01 Machine Repro. 0.03 Example of How Combined Uncertainties EffectMeasurement (Cont.)

  18. Component Estimated Max. Dev. Estimated Std. Dev. Probe Placement 0.06 0.02 Phantom Variability 0.06 0.02 HVL(+/-0.02mmAl) 0.06 0.02 kVp (+/- 1 kVp) 0.02 0.007 Example of How Combined Uncertainties EffectMeasurement Estimate Std Deviation as Max Deviation/3 (NCRP Report 112) (Cont.)

  19. Combined Uncertainty • Total estimated uncertainty • ( 1.52+ 12+ 12+12+ 32+ 22 + 22 + 22 + 0.72)1/2 = 5.2 • To estimate a confidence level of 95% use a coverage factor of 2 = 2 x 5.2 = 10.4 %

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