where all values are averages over a statistically significant
sampling period. Note that effective sweep width values are at least partially dependent
on search speed. Generally speaking, a significant increase in search speed will decrease
the effective sweep width.. W is needed to compute the search effort (Z), and Z is needed
to compute the coverage (C) based on the amount of search effort expended in the segment
relative to the segment’s physical area. The POD may be derived from the POD vs.
Coverage graph (Figure 1).
Last known position (LKP)
Last witnessed, reported, or computed position of a lost object.
Optimal Resource Allocation
The process of determining where to assign the available search resources
so that they produce the maximum possible POS in the minimum time.
Optimal Search Plan
A plan that maximizes the probability of finding the search object in the minimum amount
of time by using the results of the optimal resource allocation process.
Probable Success Rate (PSR)
The rate at which the probability of success (POS) is increased over time as the search
progresses. An optimal search plan attains the maximum PSR possible from the available
resources. PSR = W x V x Pden, where W is the effective sweep width, V is the search speed
and Pden is the probability density.
Probability Density (Pden)
The ratio of a region’s or a segment’s probability of area (POA) to its physical
area. Pden = POA/area.
Probability of Area (POA) (a.k.a., Probability of Containment or
POC)
The probability that the search object is contained within the boundaries of a region,
segment, or other geographic area. Regional POAs are generally determined by consensus and
scenario analysis. Segment POAs may be computed from regional probability densities and
segment areas.
Adjusted POA (POAadjusted) or Updated POA
The modified POA of a segment after an unsuccessful search in that segment
(POAadjusted = POAold - POS). Used to measure the decreasing probability that the search
object is in the segment. "POAold" is the POA of the segment prior to it being
searched (preadjusted); and "POS," as used here, is the product of the POAold
and the POD (preferably PODret) for this search. After multiple searches of a segment,
POAadjusted = POAinitial x (1- PODcum)
Initial POA (POAinitial)
The initial POA assigned (by consensus, ugh!) or computed (from regional Pden and segment
area) prior to any searching.
Probability of Detection (POD or SPOD to denote POD in a segment)
The initial POA assigned prior to any searching. Initial POA values must be based on a
careful and thorough evaluation of all the available evidence, data, clues, etc. pertinent
to the incident. Initial POA values, or the relative values used to compute them, must be
in the correct proportions to one another. A region with an assigned value of
"8" on a scale of 0 to 10 must be twice as likely to contain the search object
as a region that is assigned a "4." Similarly, a region with a POA of 20% must
actually be twice as likely to contain the object as one with a POA of 10%. If the
relative values used are in the correct proportions, the POA percentages computed from
them will also be in the correct proportions.
Ideally, the search area will be divided into some number of
regions based on the available evidence, data, clues, etc. which bear on where the subject
is more likely and less likely to be at the time of the first search. POA values would
then be assigned to these regions. If necessary, these regions may be sub-divided into
searchable segments. Segment POA values would be computed by prorating the region’s
POA among the region’s segments by segment area. That is, a segment one-third as
large as the region would get one-third of the region’s initial POA as its initial
POA.
As new evidence comes to light, initial POA values may be changed
if and as appropriate, regardless of how much searching has already been done. Adjusted
segment POAs may then be recomputed using the formula given above for adjusting POA after
multiple searches using PODcum.
Probability of Detection (POD or SPOD to denote POD in a segment)
- The probability of the search object being detected, assuming it was in the segment
searched. SPOD measures sensor effectiveness, thoroughness, and quality. SPOD is a
function of the coverage (C) achieved in the segment, as shown in Figure 1.
Cumulative Segment POD (SPODcum)
After the same segment is searched multiple times, the chances of detecting any search
object in the segment are increased. This increasing probability of detecting a search
object after multiple searches in the same segment is called SPODcum.
SPODcum = 1 - ((1-SPOD1) x (1-SPOD2) x ... x (1-SPODn)) SPODcum
= SPOScum/SPOAinitial
SPODcum = 1 – (SPOAadjusted/SPOAinitial)
Retrospective Segment POD (SPODret)
The POD estimated by using information obtained from debriefing the searchers to estimate
the effective sweep width, search effort, and coverage after the search of a segment.
Predictive Segment POD (SPODpre)
The POD estimated by search planners prior to the search of a segment based on predicted
values for effective sweep width (W), level of search effort (Z), and coverage (C).
Probability of Success (POS)
The probability of finding the search object with a particular search. POS measures search
effectiveness.
Cumulative probability of success (POScum)
The accumulated probability of finding the search object with all the search effort
expended over all searches to date. POScum is a measure of search effectiveness.
Segment POS (SPOS)
The probability of finding the search object in the segment on a particular search (i.e.,
during a particular operational period). SPOS = SPOA x SPOD
Segment POScum (SPOScum)
The sum of the POS values for each search in this segment (SPOScum = SPOS1 + SPOS2
+…+ SPOSn). Used to measure the increasing possibility that the search object is
outside of the segment described. This value can never exceed the initial POA value
assigned to the segment. SPOScum is a measure of search effectiveness to date in this
segment.
SPOScum = SPOAinitial – SPOAadjusted
Overall POS (OPOS)
The probability of finding the search object during a particular operational period. OPOS
= S1POS + S2POS + …+ SnPOS for the operational period of interest.
Overall POScum (OPOScum)
The sum of all individual segment POScum values (OPOScum = S1POScum + S2POScum +... +
SnPOScum). Used to measure the increasing possibility that the search object is outside of
the search area. OPOScum is a measure of overall search effectiveness.
Region
A subset of the search area used to quickly apply POA to large parts of the search area.
Regions are based on scenarios and probabilities, not searchability.
Scenario
A description of what the subject may have done and what he or she may have experienced
since he or she was last seen or known to be safe. A scenario should be consistent with a
significant fraction of the available evidence and data. Normally, multiple scenarios
should be considered, especially when not all the available pieces of evidence and data
are consistent with all other pieces.
Search
An operation that uses available resources to find persons or objects
whose exact location is currently unknown.
Search Area
The area, determined by the search planner, that consists of the smallest area, consistent
with all available information, containing all possible search object locations and
therefore all segments. This area may be subdivided into regions based on the probable
scenarios, and into segments for the purpose of assigning specific responsibilities to the
available search resources.
Search Effort
The effective sweep width (W) times search speed (V) times hours spent in the search area
(T) equals the search effort (Z = W x V x T) for one searcher or one resource (such as a
boat or aircraft and its crew). Alternately, Z = W x D, where D is the lineal distance
traveled. The unit of measure for search effort is described in area (i.e., square miles,
etc.). If multiple searchers simultaneously follow independent paths when searching and
together achieve approximately uniform coverage of the segment, then the total search
effort is given by Z = n x W x V x T where n is the number of searchers. Loosely speaking,
search effort may be thought of as the amount of area that can be (or was) effectively
swept by the searchers.
Search Object
A generic term used to indicate evidence (clue) related to a lost subject, or the lost
subject. In the same segment, different search objects generally have different effective
sweep widths (or "detectabilities"). This means that for any given search of the
segment, different coverages, and hence different PODs, will be achieved with respect to
different search objects.
Search Speed
The average rate of travel (speed over the ground) of searchers while engaged in search
operations within a segment. (V).
Segment
A designated subarea (subset of the search area) to be searched by one or more
specifically assigned search resources. The size of a segment is determined by the search
planner. The boundaries of a segment are identifiable both in the field and on a map and
are based on searchability, not probability.
Sensor
A mechanical, organic, or electronic device used to detect a search object. A human,
multi-sensor platform is often referred to as a "searcher."
Selected Inland
Search
Figure 1