Census is a survey carried out every ten years to gather population data (1). Each householder is asked to complete a detailed questionnaire about the number of people living in their home, their age, gender, employment, home and languages spoken (1).

Civil registrations of births, marriages and deaths keep an up-to-date count of the population (1). In China, National Population Sample Surveys have been conducted annually, with 1% of the population being asked to complete the form (1).

Government records: information on migration may be gathered from visa applications or Borders Agency (1). Data from electoral roll and NHS records allows population data to be updated in between census collection (1).

Accurate population data is difficult to collect in countries where illiteracy rates are high. Many people may be unable to read the form or may fill it in inaccurately. They may require help from enumerators, which costs money (1). In some countries, people speak many different languages making it expensive to issue a census to everybody. In India for example, there are 22 official languages and over a 1000 local languages (1).

Countries with large numbers of migrants, e.g. rural to urban migrants may result in some people being missed or counted twice (1). Similarly if countries have large numbers of people who are homeless, nomadic or live in shanty towns, e.g. Dharavi in Mumbai, with no official address then they will be difficult for enumerators to track down to deliver a census form (1). Mountainous terrain or poor communication links (e.g. Himalayas) can make it difficult to reach isolated villages, resulting in many people being left uncounted (1).

Carrying out a census may be largely impossible for many countries given the sheer cost involved. Training enumerators, distributing forms and analysing results can be very expensive and not a priority for countries struggling with healthcare and education (1). In addition, many people lie on their census form in fear of the government, e.g. in China, families may not record the birth of a baby girl as a result of the One-Child Policy (1). Wars, e.g. in Iraq, may also lead to inaccuracies as death rates will be difficult to monitor (1).