[ RadSafe ] Re:False alarms: was: NYC permitting of detectors

Clayton J Bradt cjb01 at health.state.ny.us
Tue Feb 12 10:31:14 CST 2008

Jim Hardeman wrote:

Clayton --

I agree w/ your conclusions, but I think you may be significantly
underestimating the effort required to determine whether an alarm is
"false". I'm not entirely certain of NYPD's CONOPS (concept of operations)
but I believe alarm adjudication requires identification of the
radionuclide(s) giving rise to the alarm. As most street cops to not carry
radionuclide identification hardware, it may be 15 minutes or more before
they can close out an alarm.

Yes. Again I think my calculation is bounding of the lower limit of
person-hours (thank you, Franz) required to resolve false positives.  The
result from your estimate multiplies my original by 15 to 2,550
person-hours per day. Assuming three 8-hour shifts working round the clock
that would require about 320 additional FTEs for NYPD.  Maybe they can take
them off parking meter duty.  There could be a real silver lining in this!

As long as I'm at it, let's consider Franz's observation that my original
assumption of 1 WMD per day plying the streets of NYC is wildly
unrealistic. Let's assume one WMD per year (also unrealistic but not as
wild). That would mean one true positive (WMD) for every (10,000 patients
per day) X (365 days per year)= 3,650,000 false positives. So a positive
predictive value of about 3E-7!  The positive predictive value is used as a
metric for the effectiveness of a particular test as a screening tool.
Imaging a medical test that required resolving over 3 million false
positives for every true positive observed!

Who came up with this plan to detect WMDs?

To paraphrase my father-in-law, this is an example of the 5P's in action:
Poor Planning Produces Poor Performance.

Clayton J. Bradt
Assistant Bureau Director
NYS Dept. of Health

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