Once a year, thousands of clipboard-toting volunteers fan out across some of the nation's largest cities on a cold winter's night.

They are tasked with a deceptively complex job: to count the number of homeless people on the streets.

The count, mandated by the federal government in order for the cities to receive certain kinds of funding, has taken place all over the country in the last few weeks.

More than 3,000 volunteers crisscrossed New York City early Tuesday to count the city's growing homeless population.

But some social service groups argue that doing it only once a year leads to an inaccurate tally, because the number of homeless changes from one night to the next.

Defenders say it provides a snapshot to help determine policy and funding.