Population counters such as the one maintained by the United Nations Population Division are estimated based on data collected by governments and are therefore not precisely accurate. Also, the UNPD only updates its population data once every five years, so the figures are not reflective of real-time changes. The data used to calculate population estimates include government censuses, independent surveys of health and demographics, official birth and death records and information and studies from various organizations such as the World Health Organization, the United Nations High Commission on Refugees and academic institutions, notes LiveScience.com.
Using census data to determine population can be problematic for maintaining accuracy, as standards of census data vary between countries, states LiveScience.com. In some countries, censuses may be taken decades apart, and the data that are gathered may be of questionable credibility. In countries with significant political and social upheaval, for example, accurate censuses are often not possible. Although data from other sources, such as the Demographic and Health Surveys organization, can help to enhance accuracy, there is still no way to be certain, reveals LiveScience.com.
Projections of population changes are also problematic because they are based on assumptions of reproductive behavior that are in turn based on observations of changes in other countries.