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Measurement UncertaintyPowerPoint Presentation

Measurement Uncertainty

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## PowerPoint Slideshow about ' Measurement Uncertainty' - airell

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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

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.)

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 EffectMeasurementEstimate Std Deviation as Max Deviation/3 (NCRP Report 112)

(Cont.)

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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