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Airport Passenger Screening Using Backscatter X-Ray Machines Compliance with Standards (2015) / Chapter Skim
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Appendix D: Summary of Statistics from Previous Studies
Pages 170-175

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From page 170...
... Paulter, Assessment of the Rapiscan Secure 1000 Single Pose (ATR version) for Conformance with National Radiological Safety Standards, NIST report for the TSA, inter-agency agreement HSHQDC-11-X-00585, April 19, 2012; hereinafter referred to as the NIST report.
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From page 171...
... This summary accurately describes what the studies say, but the studies are cryptic in discussing how they reached margins of error for their estimates of average exposure. At a minimum, all the studies deviate from standard statistical methods for describing the uncertainty in key parameters.
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From page 172...
... By contrast, the standard error reflects the uncertainty in using the average of the original measurements as a proxy for the overall mean. As the sample size increases, the standard deviation among the measurements would not be expected to change because the early pattern of spread would tend to be replicated among later measurements; however, the standard error would decrease because bigger samples produce more accurate results.
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From page 173...
... with SE = .001. The best guess, given what the authors said, is that they were reporting the standard deviation, because they state that the range from the smallest to largest observations extended from .04 µGy to .052 µGy.
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From page 174...
... However, without knowing more about the measurement procedures, there is no way to combine the results of the three studies that is manifestly correct. One approach might assume each of the three studies yielded an average exposure that differs from μ, the true average exposure for all Rapiscan machines, by an amount that follows a zero-mean bell-shaped normal curve, with the standard deviation σ of all three curves being the same.
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From page 175...
... , then the true standard deviation σ could be larger than 3. The confidence interval takes account of worst-case possibilities as well as more typical ones.
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Key Terms

  • percent confidence
  • average exposure
  • standard error
  • mean exposure
  • nist report

  • hereinafter referred
  • aapm report
  • effective dose
  • five scans
  • studies yielded

  • random sample
  • actually reported
  • random samples
  • rapiscan machines
  • authors actually

  • single pose
  • rapiscan units
  • athe value
  • aberdeen proving
  • dose equivalent

  • yielded mean
  • measurement error
  • health command
  • statistically consistent
  • normal curve

  • pose dosimetry
  • interval takes
  • engineering assessment
  • familiar measure
  • pose configuration


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