Beijing ? China kicked off a once-a-decade census Monday, a whirlwind 10-day head count that sees 6 million census takers scrutinize apartment blocks, scour migrant areas and scan rural villages to document massive demographic changes in the world’s most populous country.

And they aim to count every person.

The 2000 tally put China’s official population at 1.295 billion people, but missed migrant workers living in cities for less than six months. In the 10 years since, there has been an extensive shift in the population base as tens of millions of migrant workers have poured into urban areas looking for work.

“Wherever you are living from Nov. 1 to Nov. 10, you will be counted,” said Zhang Xueyuan, director of the publicity for the Beijing census committee.

It is the sixth time China has carried out a national census, but the first time it will count people where they live and not where their resident certificate, or hukou, is legally registered. The change will better track the demographic changes and find the true size of China’s giant cities, the populations of which up to now have been only estimates.

China has gone to great lengths to promote the census this year. In Beijing, giant, colorful banners flying across neighborhood gates have slogans such as: “The census is for the nation and each citizen,” and “Everyone participates in the census.”

Unlike the U.S. census, where residents are asked to fill out and mail in forms in a yearlong undertaking, Chinese census-takers plan to speed up the process by going door-to-door asking people questions about their education level, family history, employment situation, and resident status.

One of the first to be counted Monday in Beijing was retiree Ren Shuanggeng, who flashed a big smile to welcome census takers into his apartment in central Beijing.

Two neighborhood surveyors decked out in census vests with identification badges spared no time in reeling off questions.

“How many years have you lived here? How many people live here? Where do your children live? How old are they? How long have you been retired?”

Ren is one of more than 1.3 billion Chinese whom officials aim to question — a mammoth task considering the almost constant swirl of undocumented migrant workers on the hunt for better jobs.

Every census-taker covers about 80 to 100 households, where about 90 percent have to answer 18 questions about home ownership, jobs and family members, said Cai Jun, an official with the Beijing census committee. The other 10 percent, randomly selected, take an extended 45-question survey that seeks further information on reasons for moving, unemployment and other personal details.

“Going door-to-door allows us to be thorough so that we can survey migrant workers and others who may not have a permanent address in Beijing,” said Cai.

This is the first year foreigners, plus people from Hong Kong and Macau, will be counted, Cai said. They will be required to answer only eight questions.

One of the biggest challenges is to document China’s migrant or “floating population,” which will show the government a better picture of the numbers in its giant cities.

About 140 million migrant workers work outside of their hometowns, according to a 2009 National Bureau of Statistics report, many of whom remain unregistered.