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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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Presentation Transcript

Measurements
Measurements

  • Accuracy - hitting the center of the target

  • Precision - tight pattern of hits

  • Bias - all two inches high

  • Uncertainty - circle containing 95% hits


Traceability
Traceability

  • Accuracy - close to system limitation

  • Precision - range

  • Bias - identified and corrected for

  • Acceptable Uncertainty - 2 sigma

  • Unbroken links to national standards


Calibration traceability
Calibration Traceability

  • Unbroken linkage to national standards

  • Uncertainty - elements identified and quantified

  • Strong QC/QA program

  • Periodic MQA testing


Measurement traceability
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
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
2 cases, more than 2 results

  • Package survey of radioactive material

  • Mammography machine measurements


Package
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
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
ST-2 Measurement Guidance

Correction factors for package and detector sizes


Mammography measurements
Mammography Measurements

  • Exposure at skin entrance

  • Half value layer

  • Non-invasive kVp measurements

  • Mean glandular dose calculation







Example of how combined uncertainties effect measurement

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.)


Example of how combined uncertainties effect measurement1

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.)


Combined uncertainty
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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