They say the number of hidden homeless is twice as great as was thought but the proportion of mentally ill people within it is not as big as has been believed. However, they say that about half the homeless population has some form of mental disorder. Dr Stuart Turner, of Camden and Islington Community Health Services Trust, and colleagues used the 'capture-recapture' method accepted by biologists as an accurate means of counting species in the wild.
They explain in tomorrow's British Medical Journal how they identified an area of north-east Westminster, London, and over six months measured more than 2,150 contacts made by 1,640 homeless individuals with the statutory and voluntary agencies. They were able to estimate that the hidden homeless population for the period was 3,293, with a total population of about 5,000.
Professor Ronald Laporte, an epidemiologist at the University of Pittsburgh, United States, says in an accompanying leading article that counting people has always been a lengthy, costly process. Ecologists have long recognised that a complete count of wildlife is impossible. 'Instead, they developed intuitive estimators based on incomplete sampling: capture-recapture,' he says.
'If you want to ascertain the number of fish in the Sea of Galilee you would go out and catch fish, tag them and then release them. On subsequent days you would net fish again and note the number of tagged fish in each catch.'
By counting the tagged 'repeats' and the numbers of untagged fish, estimates can be made of the real population. In London, recording names in the first count took the place of tagging.
Dr Turner says a recent report had estimated that in the (former) Bloomsbury Health Authority boundaries that contain the study area, there were 3,467 homeless people.
'The area covered by this study was about half the size of Bloomsbury Health Authority, yet 1,640 homeless people were identified and the capture-recapture analysis estimated that there were another 3,293 homeless people unobserved.'
The researchers believe the method is a good way of counting populations which are hard to identify by other means and that 'capture-recapture' is an accurate means of estimating the provision needed for the homeless.