It's important for countries to have an accurate population figure. Governments need to know not just how many people there are but how old they are, where they live and the kind of work they're doing. This helps governments to plan health and education services, future housing needs and so on. Most use a census (usually every 10 years) to count their population.

Each country faces different challenges in collecting this data and the quality of the results varies - some countries have no data. Many countries find the cost of carrying out an effective census is beyond them.

Problems associated with collecting census information vary and can be more or less severe. They may include:

• the size of a country
• the wealth of the country – poor countries can't afford the public expenditure
• difficulties getting to some parts because of:
• the terrain – mountainous areas, poor roads or no roads
• the climate – rainy seasons frequently hinder movement
• wars – within one country or with another disrupt normal life

• communication – in some countries a number of different languages are spoken
• literacy – people who can't read or write will not fill in forms without help

Have a look at the ten Country Profiles we've chosen. Each has particular population features and census issues.