The apparent failure of census-takers to carry out their task is likely to lead to reforms of China's household registration system, demographers claim.
The census, which ended yesterday, was extended from 10 to 15 days because in many areas, the tallies failed to match those in the household registration data kept in local police stations.
Mainland demographers and statisticians discount stories that the census will reveal a larger population than those used in current estimates.
'There will be no suppression with the aggregate data - the population may even be smaller than the published data,' said Zoo Xuejin, a Shanghai Academy of Social Sciences demographer.
'On the local level there is under-reporting of births but on the provincial and national level, the data is always upwardly adjusted by the State Statistical Bureau,' he explained.
While the existence of some children at birth is not recorded, once they grow up and go to school or enter employment, their existence then generally is entered in official data.
More than exposing 'black births', the census confusion has revealed the failure of China's almost unique household registration system.
In most countries, a census records details of individuals at a given moment irrespective of where they are.
China's census records people at the permanent address of their household, irrespective of where they actually live or may be at the time of the census. Those who have left their homes for longer than six months are expected to register in their new residential area. If they have been away for less than six months, they are expected to go home and be recorded in their household. This means the census is effectively little more than a check on the household residency. Census-takers rely on data from work units, police stations and local committees to locate households.
'In the 1990 census, we found that Beijing population was exactly as that recorded by the public security departments to within a few tens of thousands,' said Gu Yenzhou, a statistician in charge of Beijing's census work.
As police use harsh measures to enforce residency laws to prevent rural dwellers from migrating to cities, tens of millions have an incentive to avoid complying with the head count. 'If people want to avoid taking part, it is very difficult for us to do anything about it,' Mr Gu admitted.
The vast increase in mobility seen in the 1990s has undermined the residency system as more and more people have reasons to manipulate it.
For instance, those who moved from Beijing's centre to suburbs have kept their original residency to ensure access to good schools for their offspring. The current effort to discover the nature of this 'floating population', which some say totals 120 million, now looks doomed. Even in tightly controlled Beijing, about 2.9 million are not living in their registered domicile.
Given the difficulties, yesterday's Beijing Youth Daily even carried suggestions that China should adapt the practice of other countries and allow people to live where they wish.
Instead of the police enforcing household registration, China should switch to the reliance on identity cards or social insurance numbers for most bureaucratic or legal purposes.