More than 1 billion Chinese citizens will participate in a once-in-a-decade census that will inform policymakers on demographic shifts in the world's most-populous but fast-ageing nation, and help shape social and economic policies in the years to come. Over 7 million census takers began going door to door on Sunday to collect information ranging from car ownership to whether they have family members in Hong Kong or Taiwan. Census takers, instead of pen and paper, are armed with smartphones and computer tablets.