ESTIMATING NUMBERS OF HOMELESS AND HOMELESS MENTALLY-ILL PEOPLE IN NORTH-EAST WESTMINSTER BY USING CAPTURE-RECAPTURE ANALYSIS

Objectives-To use routinely collected data to provide a reliable estim ate of the size and psychiatric morbidity of the homeless population o f a given geographical area by using capture-recapture analysis. Desig n-A multiple sample, log-linear capture-recapture method was applied t o a defined area of central London during 6 months. The method calcula tes the total homeless population from the sum of the population actua lly observed and an estimate of the unobserved population. Data were c ollected from local agencies used by homeless people. Subjects-Homeles s people in north east Westminster residing in bed and breakfast accom modation 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, g iving a total homeless population for the period of around 5000 (SD 12 50). Mental health problems were significantly less prominent in the u nobserved 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 problem s of ascertainment in estimating populations of homeless and homeless mentally ill people. Prevalences of mental illness derived from survey s that do not correct for ascertainment are likely to be falsely infla ted while at the same time underestimating the total size of the homel ess 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 w ithin health services.