Posts Tagged ‘Missing Person Behaviour Statistics’

Elements of the Optimal Search Problem

January 19th, 2010

Lawrence Stone defined the elements of the optimal search problem in his 1986 book, Theory of Optimal Search.

This was paraphrased extremely well by Cooper, Frost and Robe in their 2003 report – Compatibility of Land SAR Procedures with Search Theory as quoted below;

A probability density distribution on search object location and state (so the probability of containment, POC (a.k.a. POA for “probability of area”), for any subset of the possible locations and states can be estimated),

A detection function relating the probability of detecting (POD) the object if it is in a searched area to the density of the searching effort expended there,

A known finite amount of available searching effort, and

An optimization criterion of maximizing probability of finding the object in a desirable state (probability of success or POS) subject to the constraint on effort availability.

I will endeavour to simplify further. In order to need and/or use the mathematics of search theory you require four essential elements.

  1. The ability to predict the likelihood that an object is in any particular search area or region. This might be done using sophisticated computer software working with the latest missing person behaviour statistics, or could be as simple as a coming up with a consensus within the search planning team.
  2. The ability to calculate the likelihood a given search resource will have of finding the object if it in the area being searched – unfortunately we can’t ask how many clues would you have found! [See my definition of POD for a brief explanation of why]
  3. A limited but known amount of search resource – when do you ever get too much search resource?
  4. A method of calculating the best way to use the search resource to maximise the chances of finding the search object as quickly as possible.

I’ll look in detail at each of these in further posts.

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Probability of Area (POA)

January 8th, 2010

Probability of Area (POA) refers to the likelihood of the object being searched for actually being in the area.

There are a number of ways of calculating the POA; either through subjective reasoning [coming up with a consensus] or through the use of existing statistics and models – such as missing person behaviour statistics.

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Missing Person Behaviour and SAR Callout Study

January 7th, 2010

After several years of messing around I have finally decided – with much prompting and encouragement from others – to get around to studying missing person behaviour within the lowland search environment.

ALSAR and LSDog teams deal with only a tiny fraction of all missing persons and indeed deal with only a small fraction of the types of misper that the majority of SAR teams deal with. The majority of lowland search callouts are for Despondents, Mispers with Dementia and Mispers with Mental Health Issues of one form or another. Likewise, the search environment is often very different – much more urban and sub-urban searches – very few wilderness searches!

As such the value of any data collected by these lowland search teams would be invaluable – to them and to the police who in the UK have the statutory duty for missing persons. Collection of such data has, in the past, been problematic. I have, therefore, recruited some volunteers who have indicated a willingness to collate such data.

As a pilot, I have asked these volunteers to do a retrospective look at their 2009 callout data. This will allow for a thorough testing of the form, and give them an idea of what data needs to be calculated/kept for future searches.

The form itself is based upon the ISRID form, and indeed any data collected will be shared with ISRID. However, several suggestions have also been incorporated into the form. These include tracking all types of callout – not just misper searches; multiple misper categories - often it is difficult to place them in one category; more detailed terrain and find location boxes and so on.

The form was put in an excel format – to allow collation of all the data from the year, and for ease of recording. Initial reports suggest it works, although the form is quite complex.

I will, of course, be sharing any data collected freely.

If you are interested in the study, or wish to collect data from you team, you can either download the excel form –  ALSAR Stats Form, or contact me for more information.

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Do the police suspend vulnerable missing persons searches too quickly?

October 24th, 2009

I awoke this morning with the need to write another quick post here – on the suspension of searching. I obviously discussed it slightly yesterday with the idea that we should search areas where outstanding high risk mispers could be; however, my recent revision of missing person behaviour statistics has obviously made me think more about this.

I want to start, however, with a quote that many find surprising.

As a general principle, searching should take place throughout the day and night. This will be determined by the circumstances but in all cases where a full managed search is undertaken, there must be valid reasons why it is not continued throughout the full 24 hours.
ACPO Guidance on the Managment, Recording and Investigation of Missing Persons, 2005

How many ALSAR Units’ local police expect them to continue searching for a full 24 hour period? How many Units are asked to return for a second day?

It seems to me, that often the police attitude is that they “must” do a search and “clear” a few high probability areas to show that they have done something. There seem to exist a large number of those dangerous rationalisations that we teach our on our basic search courses within the police. [That sounds very harsh - there are a number of very, very good police officers who are dedicated and passionate about finding missing persons in the same way that search volunteers are - And I do recognise this!]

But the reality is the police very often run out of the will (and a great deal of this is about the political will to carry on search operations that are very man-power intensive), the man-power and the budget to actually do more than this initial search. A quick look at the ALSAR stats shows how few searches actually result is a find. Many are well outside the area and still mobile, but there remain still a few potentially still alive, lying just outside our search areas. How do I know?

Take a look at Koester’s Lost Person Behavior - Survivability ;

Dementia – just 5% will die within 24 hours, 35% will die over the next 48 hours or so but 60% will still be alive at this point. Even after 96 hours, 46% will still be alive.

Despondents – 25% will kill themselves within 24 hours, but there will still be 19% alive after 72 hours.

Koester writes a very clear warning about using these statistics;

Survivability statistics must not be the major criteria for deciding to suspend a search… It may be appropriate to use survival statistics as the sole reason to prolong a search.
Lost Person Behavior, 2008

And I think my thinking neatly ties in with this last sentence – should we start using these survival statistics to persuade our local police to continue search operations for longer than our present 4-6 hour searches?

Should we start expecting searches to last 2 or 3 operational periods if nothing is found and planning accordingly?

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