PROBABILITY OF DETECTION (POD) - PART 1
by John Mindock
This article originally appeared in Volume 2, Issue 9 of Lost... and Found.
This article was first published in September of 1997, and definitely contains information that does not reflect the current state of the art of search management. Please read the editorial notes that have been added more recently!
INFORMAL DEFINITIONThe probability that your team noticed the subject in the area you searched. Also can be depicted as the percentage of the assignment area (big enough for a person to hide in) that you actually looked at while searching. It is NOT a measure of the quality of a team's effort, nor of their proficiency as searchers.
[Ed. Note added 27 Feb 2006: Formally, the POD is the conditional probability that search of an area using a specific technique would locate the search object given that it is there, or equivalently, the probability that a given search technique will detect search objects while traveling through a swarm of randomly distributed, identical objects. See also our more recent article on Search Theory]
OVERVIEWDuring a debriefing after an assignment, field teams are commonly requested to report their `Probability of Detection'. Often this is divided into two types - Responsive and Unresponsive. Often, the POD is formulated by using `gut feeling' based on experience, wishful thinking, and some idea of what was expected in the first place. Note - POD's higher than 80% are defacto counted as 80%, perhaps due to legal concerns.
[Ed. Note added 27 Feb 2006: This technique of gathering subjective POD estimates from field teams has been called under question recently, and is generally considered to be a poor substitute for more rigorously defined and computed POD values. The previous sentence is in fact a kind understatement of the way this technique is viewed by professionals in the SAR business. See also our more recent article on Search Theory and the references it contains.]
UNRESPONSIVE PODUnresponsive POD is simple to fathom - if the subject was lying within your search area, but unable to respond to you, what's the chance that you saw him? This usually is a function of the size of the area, the number of searchers, the rate of travel, the terrain/flora, the subject's clothing, and the thoroughness of the search tactics. Weather and darkness also can figure into the calculation. A subject may be unresponsive because of death, unconsciousness, weakness, fear of searchers, or evasion.
RESPONSIVE PODAssuming that the standard attraction techniques (yelling, whistles) are used, what is the probability that the person was in the area you searched, and yet you did not hear him responding to you? This measurement is a function of the size of the search area and the number of searchers. Rate of travel, terrain/flora, and thoroughness are not really factors one way or another. Weather (such as high winds) may figure in, but darkness has no bearing. Responsive POD is never less than unresponsive POD, for obvious reasons.
SIGNIFICANCE OF POD TO THE MISSION MANAGEMENTSubsequent to the `initial attack' portion of a mission, the territory being searched is subdivided into portions called `search segments'. After a certain amount of effort over several operational periods, mission management develops a strategy where they start eliminating search segments. This is often used to justify a decision to suspend the mission. The criteria for suspending a mission are somewhat subjective, but for this paper let's say that much of that decision is based upon reaching 75% POD in all segments within the subject's probable range of travel. This range is determined by statistical analysis of many missions nationwide with similar subjects, as described in documents used by Incident Management for planning the search. At this point, the teams being deployed notice a change in assignment directives, hearing instructions like `search this segment to a 80% POD' instead of `go up such and such trail'. The nature of POD has shifted to a pre-deployment specification, as opposed to an assessment of the results upon return.
The decision to suspend a mission will occur after a number of operational periods, and the subject will have been in the elements for a significant time. The assumption at that time will be that the subject is no longer able to respond. During this phase of a mission, the assignment to search a segment at `x' POD really means an unresponsive POD. This normally goes unsaid during the team briefings, but it is the only sensible interpretation, i.e., if the person is still highly likely to be responsive, we wouldn't dare consider suspending the search yet.
MANAGEMENT USAGE FOR RESPONSIVE PODThere are scant few scenarios where this is used to make management decisions. Perhaps if a team had not used common attraction techniques (whistles, etc.), or if there had been a howling wind, a low responsive POD might instigate a re-search of the area. Another scenario would be where searchers spread out too far to hear each other and the subject might be in-between. If your team returned with a very high responsive POD in the early stages of a mission (when the subject might still be likely to be responsive), mission management might decide to send teams into other high-potential areas instead of re-searching your area in an `area search' mode.
MANAGEMENT USAGE FOR UNRESPONSIVE PODFrom a practical standpoint, unresponsive POD is more likely to have an influence on management decisions than responsive POD does. A search segment can be eliminated from further searching if the area has been covered to a high unresponsive POD. There is a somewhat rigorous set of cascading mathematical calculations that depict the actual cumulative POD's and the prescription for elimination. As mentioned above, the nature of the assignments change to where a high unresponsive POD is requested prior to deployment. This dictates the search tactics, and the thoroughness to be applied.
EFFICIENCY VS. THOROUGHNESSIn previous paragraphs, it was mentioned that thoroughness is a factor in POD. Lack of thoroughness in this sense does not imply sloppiness, laziness, or some other less-than-desirable behavior. Thoroughness and efficiency are different aspects of the search spectrum, and neither is right (or wrong) by itself. Efficiency refers to searching the largest territory in a minimal amount of time, using limited personnel resources. Thoroughness means ignoring time/personnel constraints and looking `everywhere'. In general, efficiency implies rather swift passage while thoroughness denotes a slower pace. Hasty teams are intended to perform `efficient' searches as opposed to `thorough' ones. Looking behind every bush, rock, and log (in an effort to be extremely thorough) is contradictory to the theory of using hasty teams, and represents improper execution for their assignment. Since hasty teams are responsible for 80% of all finds, it is important to perform the proper techniques during such an assignment.
A corollary to this is that a low POD is acceptable (indeed, often expected) of hasty teams. On the other hand, `area search' teams are expected to be thorough, and generally are counted upon to return with a very high POD. Failure to return with a high POD may require the segment to be re-searched.
SELF-QUIZ ON POD - PART 1
- When do teams report POD to Incident Management?
- What is `Responsive POD'?
- What are some factors that determine Responsive POD?
- What is `Unresponsive POD'?
- What are some factors that determine Unresponsive POD?
- What does Incident Management use POD for?
- Which type of POD is the most pertinent to a decision to suspend a mission?
- At what stage of a mission is POD likely to be a pre-assigned criteria?
- When is it desirable to be less-than-thorough on a search assignment?
- What is the relationship between the POD and the quality of a team's efforts?
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