President Xi Jinping answers census workers' questions in Beijing on Monday. On Sunday, China began its seventh national census, an effort that is expected to continue through Dec 10. JU PENG/XINHUA

President Xi Jinping underscored on Monday the importance of the national census in formulating economic and social development policies of the Party and the country, calling it an issue of overall strategic significance for the world's most populous country.

Xi made the remark after he took part in the seventh national census in Beijing. He extended warm greetings to the 7 million census takers who began going door to door to document demographic changes in China starting on Sunday.

As China is faced with pressure from a large population as well as challenges from demographic transitions, Xi called for a thorough investigation of China's population size, structure and distribution as well as trends in China's demographic change, Xi said.

The census, a major survey of national conditions, also is a big event in the work of the Party and State, said Xi, who is also general secretary of the Communist Party of China Central Committee.

With accurate statistical information provided by the census, China can better improve its strategies and policy systems for population development, formulate plans for economic and social development and promote high-quality economic development, he said.

China conducts a national population census every 10 years.

Xi said that the large amount of detailed demographic data obtained from the previous six censuses since the foundation of the People's Republic of China had played an important role in the country's policy formulation.

He also called on the whole of society to be understanding and supportive of the census takers, who need to pay close attention to their work given the current COVID-19 pandemic prevention and control measures.

The census takers started their work at midnight on Sunday as profound demographic changes in China－including a rapidly aging population and shrinking workforce－are expected to have a tremendous impact on China's economic and social development in the coming years.

The effort comes on the heels of the release of the blueprint for the country's development over the next 15 years.

"The seventh national census will provide the most complete, accurate and scientific data support for China to improve its strategy for balanced population development over the long term, create national economic and social development plans and build itself into a prosperous modern country," said Zhai Zhenwu, president of the China Population Association.

Although China is still the world's most populous country, the rate of the nation's population growth has been declining since 2010, and the trend will continue and produce far-reaching effects on the economy and society, Zhai said.

The birthrate on the Chinese mainland fell to 10.48 per 1,000 last year－the lowest ratio in 70 years, according to the National Bureau of Statistics.

The number of births dropped last year by 580,000 year-on-year, marking the third consecutive year of falling births despite the adoption of the universal second-child policy at the beginning of 2016.That policy encourages all couples to have two children to counter an aging population.

Population experts both at home and abroad have predicted a continued drop in births and a decline in China's population within a decade because of factors such as young people's reluctance to have more children and a smaller number of women of childbearing age.

By the end of the century, China's total population may fall to 732 million, just about half last year's 1.4 billion, which would make China the third-most populous country after India and Nigeria, according to a study published in The Lancet medical journal in July.

Zhai, the population expert, said that a shrinking workforce and acceleration of the aging of the population resulting from a decrease in births will cause major challenges for China's economy and society.

Between 2010 and 2018, the number of people age 60 or older in China rose from 178 million to 249 million, and that age group's share of the total population increased from 13.3 percent to 17.9 percent, he said.

In addition, other aspects of the population, including its distribution, education and marriage and occupational status, have undergone tremendous changes over the past decade. The full picture will be disclosed by the census, Zhai said.

Wang Guangzhou, a population expert from the Chinese Academy of Social Sciences, said China's rapidly developing big data technology could be used to improve the efficiency and accuracy of the census.

New technologies such as big data have been used to facilitate the census, which involves collecting information like the name, ID number, gender, marital status, education and profession of Chinese citizens.

Zhao Bo, a census taker in Changzhou, Jiangsu province, did not get an answer at one apartment in a residential building in the Qingguoxiang community during the door-to-door household visits.

Zhao then scanned a QR code on the electric meter with a mobile app supported by the State Grid Changzhou Power Supply Company. It showed a red code, telling Zhao that no one was living in the apartment.

"In the past, when there was no answer at certain apartments, we had to make repeated visits, ask their neighbors to contact the apartment owner or even determine whether anyone lived there by dust on the door," Zhao said.

"Big data helps us find out whether anyone lives in an apartment or not, and thus saves us much trouble and improves census efficiency and accuracy," said Yuan Haijun, who works at the census office in the city.

Xinhua contributed to this story.