BEIJING (Reuters) - 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. Authorities will publish the survey results in April. Below are some changes in the way the census is being conducted compared with the 2010 survey, to reflect a wealthier and older population: