BMJ 1994;308:27-30 (1 January)

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Estimated numbers of homeless and homeless mentally ill people in north east Westminster by using capture-recapture analysis

N Fisher, S W Turner, R Pugh, C Taylor 

Pathfinder Community and Specialist Mental Health Services, Springfield Hospital, London SW1 7DJ Camden and Islington Community Health Services NHS Trust, Hampstead Road, London NW12 2LT National Addiction Centre, Institute of Psychaitry, London SE5 8AF.

Abstract

Objectives : To use routinely collected data to provide a reliable estimate of the size and psychiatric morbidity of the homeless population of a given geographical area by using capture-recapture analysis.
Design : A multiple sample, log-linear capture-recapture method was applied to a defined area of central London during 6 months. The method calculates the total homeless population from the sum of the population actually observed and an estimate of the unobserved population. Data were collected from local agencies used by homeless people.
Subjects : Homeless people in north east West-minster residing in bed and breakfast accommodation and hostels or sleeping rough who had contacted statutory or voluntary agencies in the area.
Results : 2150 contacts by 1640 homeless people were recorded. The estimated unobserved population was 3293, giving a total homeless population for the period of around 5000 (SD 1250). Mental health problems were significantly less prominent in the unobserved compared with the observed population (23% (754) v 40% (627), P<0.0001). For both groups the prevalence varied greatly with age and sex.
Conclusions : Capture-recapture techniques can overcome problems of ascertainment in estimating populations of homeless and homeless mentally ill people. Prevalences of mental illness derived from surveys that do not correct for ascertainment are likely to be falsely inflated while at the same time underestimating the total size of the homeless mentally ill population. Population estimates derived from capture-recapture techniques may usefully provide a good basis for including homeless populations in capitation calculations for allocating funds within health services.

implications

  • implications

  • Homelessness is associated with ill health

  • Current estimates of the size of homeless populations are unreliable because of problems in identifying all homeless people for a given time period

  • Capture-recapture methods overcome problems of ascertainment by calculating the size of the unobserved population.

  • This study estimated the population of unobserved homeless to be twice the size of the observed population

  • Mental health problems are significantly less common in the unobserved or hidden homeless compared with those who are easily surveyed.


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