*** Shifting Probability of Area Formula ***

To: 	 ldk@indigo.ie
From:    sarinfo@istar.ca
Subject: Re: search and rescue
Date: 16th March 1998

Hi IDK,
Shifting Probabilty of Area is covered in the Emergency Response
Institute's 'Search is an Emergency' publication. The formula is given
by two methods, Greg Shea's and Don Coopers:

Greg Shea's Method:

Step 1: Adjust POA
For any segment:   POA* = (1 - PODcum)POAold

Where:
cumPOD = the cumulative POD for all searches of that segment
POAold = the previous POA for the segment


Step 2: Normalize POA:
Add up all the POA* for each segment
(this Normalization Factor S, will be less than 1).
As the sum of all POA's , must add up to 1 (100%POA) we divide each
POA* by S to give the normalized (shifted) POA:

       POA shifted = POA*             for each search segment.
                     ----
                      S  

Reference: NASAR's 'Response' magazine May/June 1988


Don Cooper's Method:

New POA of Searched Area:

                                    POA(a)old x (1 - PODcum)
POA(a)new =       ---------------------------------------------------
                        [POA(a)old x  (1-PODcum)] + (1 - POA(a)old)

Where:

POA(a)old = the old POA of the segment just searched

POA(a)new = the Shifted (new) POA of the segment just searched

PODcum    = the cumulative POD of all the searches in this segment


New POA of areas NOT searched:

                         1 - POA(a)new
POA(q)new =        ----------------------  x POA(q)old
                         1 - POA(a)old


Where:

POA(a)old = the old POA of the segment just searched

POA(a)new = the Shifted (new) POA of the segment just searched

POA(q)old = the POA of the segment NOT searched before segment 'a' was
            searched

POA(q)new = the Shifted (new) POA of segment not searched before
            segment a' has been searched

Reference: Search is an Emergency, Fourth Edition 1995
           Emergency Response institute, Olympia, Washington State, USA