Only a quarter of migrant workers in the cities were covered by pensions in 2010, compared with four-fifths of locals, according to Albert Park of the Hong Kong University of Science and Technology.

ECONOMIST: China is beginning to face up to its pension problems

The stimulus package got most of them back to work quickly, but a lot of those migrant workers stayed in the interior cities because wages had improved along with working conditions.

FORBES: Is China The Next EU?

Migrant workers often can't register in the cities where they work, and subsequently can't qualify for social services such as public education or social security.

CNN: China's workforce at a crossroads

Two groups are particularly eager to avoid the census takers: migrant workers, who usually lack permits to live in their new cities, and so-called "black children, " who were born in violation of China's one-child policy.

CNN: ASIANOW - TIME Asia | The Numbers Game