NEW YORK (AP) — Over the past few weeks, thousands of clipboard-toting volunteers have fanned out across some of the nation's largest cities, tasked with a deceptively complex job: counting the number of homeless people sleeping on the streets.

After briefly speaking with homeless persons, Alexis Sypek, left, and Victoria Parker, working with The Robin Hood Foundation, check their notations during a count and survey of homeless persons on the streets of New York early Tuesday, Feb. 9, 2016. http://syndication.ap.org/AP.Distro.ContentBroker2/ContentBroker.aspx?contentid=42b9fb6f4115f6078f0f6a7067006bd0&iid=75149d95bb33492eb5a32d546b214a1a&rsn=0&recordid=75149d95bb33492eb5a32d546b214a1a&filingId=7b534603dbb948d3a553e0f71fe01059&role=Preview&reldt=2016-02-09T14:38:02&media=Photo&sz=&dest=ak&trF=NYCR112&ofn=APTOPIX%2bCounting%2bthe%2bHomeless.JPEG&fmt=jpg&relativeUrl=jpg/2016/201602/09/42b9fb6f4115f6078f0f6a7067006bd0.jpg&s3Key=preview.jpg&authToken=eNotizEOwyAQBF8EOsB3mAIpXwkGpCtiLDCKi3t8KKKRptlZKU8khwEWLngAsCZ4OThuNoWaqKrNGFSVwKu9QlX09kArpJRBGud1B3QyRzzaeXdO8259vNIcfJaxNDiXro%2f2kckrNkBOrm%2f8DwZB%2bIpoNWlrrN7xB4ejLVI%3d Over the past few weeks, thousands of clipboard-toting volunteers have fanned out across some of the nation's largest cities, tasked with a deceptively complex job: counting the number of homeless people sleeping on the streets.

They've had to do it under difficult conditions that some social service groups say are bound to produce an inaccurate tally. The official counts are done once a year, in the dead of winter, when homeless people are more likely to be hunkered down in places that are hard to see.

The challenging nature of the count was on display at 2 a.m. Tuesday in New York City when volunteers knelt down to question a man sleeping on a Manhattan sidewalk beneath a camouflage blanket. Beside him, a cardboard sign read "Anything helps."

"Excuse me? Sir?" volunteer Victoria Parker prodded, appearing to startle the man.

"Do you have some place where you consider your home or some place you live?"

"Have you served in the armed forces?"

To each question the man simply shook his head "no."

Finally, she asked if he would like to go to a shelter. Again he shook his head. She thanked him and, as her partner recorded the answers, the man laid his head back down to sleep as snow began to fall.

The count is mandated by the federal government for cities to receive certain kinds of funding. It has taken place all over the country in recent weeks, including in Philadelphia, Houston and Boise, Idaho.

Some social service groups argue that the counts should be done more often, because the number of homeless spotted on the street could change depending on how cold it is. A number also argue that the volunteers don't often venture into dark parks or remote locations — such as bridges, highway embankments or subway tunnels — and that the homeless are inherently transient and easy to miss during even a comprehensive survey.

New York's annual count usually puts the number of street homeless in the low 3,000s, but advocacy groups have suggested the actual total could be nearly double that. This year's count won't be known for months.

"It's a flawed measure and using it to make a comparison from one year to the next is deeply problematic since so many variables change," said Giselle Routhier, policy director of the Coalition for the Homeless in New York. "Doing it one day a year doesn't provide accuracy. And if you don't have an accurate read of the problem, you can't accurately identify solutions."

File photo of a homeless man making his way through the Brooklyn bridge during low temperatures at Lower Manhattan in New York Thomson Reuters In New York, the city is too vast to cover fully in one night. So, a mix of city blocks with a high density of people and those of low density are chosen for the survey based on existing data and the results are inserted into a formula to extrapolate a tally.

Even some of the groups involved acknowledge that the system is far from ideal.

"It a good thing to do but also an imperfect system," said Eric Weingartner, a managing director of The Robin Hood Foundation. "It's such a fluid population that you're never going to know what you're going to find day in, day out. But it does give the city some snapshot."

Officials suggest that the one-night approach is the best that can be done considering current fiscal and manpower restraints.

In Dallas, a renewed effort was made "to canvass the whole area, not just known encampments," in order to get a more accurate total, said Cindy Crain, chief executive officer and president of Metro Dallas Homeless Alliance, the nonprofit that coordinates the survey of the homeless.

"They will go into a Waffle House, look behind commercial buildings, back in the Wal-Mart parking lot where you may see a car with all of their stuff in it," said Crain before Dallas conducted its survey last month. She says she asks volunteers to think, "It's cold tonight. Where would you go?"

In some cities, the count comes at a time of increased scrutiny of homelessness. The Los Angeles City Council in November declared a homelessness crisis, paving the way to allow people to sleep on sidewalks and temporarily live in their cars while it continues to seek ways of housing an estimated 26,000 transients.

Stacie McDonough, 51, eats lunch by her tent in a homeless RV and tent encampment near LAX airport in Los Angeles, California, United States, October 26, 2015. REUTERS/Lucy Nicholson The city's homeless population has increased more than 10 percent over the past two years. In September, officials announced they planned to spend $100 million to eradicate homelessness.

"Capturing the data helps us advocate for funding and helps us allocate that funding so the money goes to the right places," said Naomi Goldman, spokeswoman for the Los Angeles Homeless Services Authority, which coordinated a three-day count in January. "It helps us identify trends and issues."

Housing of Urban Development Secretary Julian Castro, whose agency doles out funding contingent on cities doing the survey, joined Mayor Bill de Blasio for New York's count. De Blasio, whose administration is battling a rise in homelessness, walked several blocks and assured the homeless he encountered that the city was trying to help.

"It's very, very sad to see, especially in the middle of a city with so much wealth," de Blasio said. "This is not the way it should be."

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Associated Press writers Christopher Weber in Los Angeles, Jamie Stengle in Dallas and William Mathis in New York contributed to this report.