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 Bias - identified and corrected for Acceptable Uncertainty - 2 sigma Unbroken links to national standards

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

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

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

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

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

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

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

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

X-ray vs. Mammo Probes

Phantom Variability

Temperature Effects

HVL Measurements

Non Invasive kVp Measurements

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

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

Combined Uncertainty Total estimated uncertainty ( ) 1/2 = 5.2 To estimate a confidence level of 95% use a coverage factor of 2 = 2 x 5.2 = 10.4 %