SINCE the financial crisis of 2008 the number of people sleeping rough on the streets has more than doubled, according to charities' estimates.
But besides the highly visible vendors of street newspapers, the homeless often try to avoid attention and stay hidden. Those without a fixed address are not covered by any conventional census. So how exactly do countries measure their homeless populations?
It partly depends on the definition of homelessness. There are two categories: "rough sleeper", meaning someone who sleeps on the streets; and "statutory homelessness" for people who are in temporary accommodation or otherwise at risk of losing the roof over their head.
America, Ireland and Sweden use the same two categories under different names. But no international standard exists. Italy has no exact definition. Norway combines the two groups. In New Zealand and France anyone without habitable accommodation is considered homeless.
The best way to count rough sleepers is to go and find them. Los Angeles and New York recruit volunteers to comb the streets. In New York this happens every year; in Los Angeles every two years. London is particularly good at keeping track. Every day charity workers try to find the city’s homeless. Some ride the night buses.
To avoid double counting, the outreach workers take homeless people's names, waking them up to do so if necessary. Lack of English, use of drink and drugs and suspicion of the authorities all make it harder to get this information. Finding those in precarious or uninhabitable accommodation is more difficult still, as Britain keeps a record only of those who apply to the government for help.
There is likely to be a considerable number of so-called "hidden homeless", people living in squats or temporarily staying with friends or family. For these couch-surfers no reliable estimates exist.
The European Union attempted to introduce a standardized method of counting the homeless as part of a census in 2011. Only Poland followed the guidelines, counting both those sleeping rough and those in temporary accommodation. Going to find rough sleepers is expensive and time-consuming so most governments simply don’t bother. Paris has not kept a record since the mid 2000s.
While snapshot counts like that of Los Angeles provide only a limited amount of information, the length of time people spend on the streets is critical. Even with surveys it is inevitable that some people are being missed. But things are getting better: part of the reason for the dramatic increase in the number of rough sleepers, is that more people are going out to find the homeless than ever before.