Many Chinese scoff at such statements. The grandmothers who know the alleys of Daxing best say there's no way that migrants are going to voluntarily admit the real number of children they have when it could mean fines or worse. Company bosses in Beijing laugh about the employees they have who live and sleep where they work, and wonder how any census-taker is going to know where to look.
In an online poll on the popular sina.com website, one-third of respondents said they weren't comfortable letting census-takers into their homes.
In particular, winning trust among a migrant population that feels persecuted and discriminated against won't be easy. The answers you get from those living on the outskirts of Beijing can be evasive, and sometimes change with repeated asking: One labourer says he lives in Daxing with six family members. Half an hour later he says the real number is five, because one of his daughters recently moved to nearby Hebei province. The landlord of the same building - really just another alleyway with families behind each door - professes not to know whether it's four families or five that live in his rooms.
Even if everything goes the way the ruling Communist Party plans it, Mr. Fang acknowledged there is a margin of error to any survey of this size. And at plus or minus 1.8 per cent in the last census, there's room for an "oops" of 25 million people either way.
China's censuses were once simpler affairs. Those taken in 1953, 1964 and 1982 are seen as extremely accurate because of the level of control the government then exerted over its citizens. But the opening up of the country's economy gave birth not only to new wealth and massive migration, but also a sense of both privacy and of the value of time. "Now time is money for people. They tell us 'I don't have time to talk to you.' There's also a sense of privacy, as in 'Why are you asking me such questions?' " said Zhai Zhenwu, the director of the School of Sociology and Population Studies at Renmin University and a member of the nine-person committee that designed the census. The 1.8-per-cent margin of error in the 2000 census was 30 times that of the 1982 count, he said.
Those running the head count have noble intentions. The questions were designed, they say, to help the central government get a better hold on where people really live, what their economic conditions are and how many children in each area need to go to school.
The social policy implications for China's leaders are as staggering as the numbers. If there are many million more people than previous believed, and if most of those extra citizens are living on the fringes of society in China's cities, then the argument to reform the country's hukou registration system - which condemns migrants to society's margins by denying them access to public services outside of their registered hometowns - becomes that much more compelling.
The census will also likely give more ammunition to opponents of the country's controversial one-child policy - which has been thrust unfavourably back into the limelight in recent weeks by the story of a woman in the southern city of Xiamen, eight months pregnant with her second child, who was forced by officials to have an abortion.
Prof. Zhai said another key goal of the census is to get a more accurate picture of the country's income gap, which has expanded dramatically over the past two decades. According to the government's own figures, the Gini coefficient, which is used to measure income inequality, a decade ago passed 0.4, historic danger point for triggering instability, and now stands at 0.47. "If the census shows that the Gini coefficient is bigger [than expected] the government needs to put more effort into solving the problem. But if it shows the number is still bearable, then we can continue our current policies," Prof. Zhai said.