The main purpose of population projection is to provide estimates of future population size and structure for decision-making. The projections are vital for planning and budgeting by state and non-state stakeholders in the development community, and a well-aimed estimation is essential to making adequate preparation for future demands on resources. Besides, concerns about the localized consequences of human population growth and demographic changes have motivated forecast about the demographic future. Such concerns have necessitated projections to determine both size and structure, especially by sex and age, of future population trends in a geographical region at a particular time.
Demographers mainly view population projections as indicative rather than predictive. The methodology of a projection can either be objective or subjective  and there are several methods to make population projection . Projections are, in general, based on certain assumptions; hence population projections are the implications of assumptions at future dates as they show a picture of the future population if specific assumptions hold. Nevertheless, they do not predict whether the assumptions will hold. Future events prove the accuracy of a population forecast, as forecast represents an analyst's most realistic prediction of the future population. Projections are conditional as they are the combination of methodological assumptions, available records, and techniques.
Agencies such as United Nations Population Division (UNDP), U.S. Census Bureau, International Institute for Applied Systems Analysis (IIASA) and the World Bank make projections for global, regional and national populations, with United Nations projections being the most widely used. Also, various national statistical bodies make projections for their countries.
Nigeria’s first modern and carefully planned nationwide census was the 1952/1953 enumeration while the most recent population census was conducted in 2006. Later population figures have been estimated or projected by statistical and population agencies at national and subnational levels. Since its establishment in 1989, the National Population Commission (NPopC) of Nigeria has been tasked with collecting, analyzing and disseminating demographic data in the country. In 2000, the commission received the mandate to “review population policy, coordinate and monitor all population management activities”, and has been responsible for making the national population projections and supporting agencies at subnational levels.
Currently, Nigeria is projected as the most populous country in Africa and the 7th largest population in the world. It is projected that Nigeria will be the 3rd largest nation in the world by 2050. Population estimates at subnational level are based on the 2006 census and annualized population growth rates experienced between the 1991 and 2006 censuses at the respective level. For example, Sokoto State’s population was enumerated at 3,702,676 in the 2006 national population census and projections are based on 3% annual growth. On this basis, the population was estimated to be 4,309,896 in 2011 and 5,313,527 in 2017 by the state ministry of planning.
There are challenges with this method of projection. The projections are based on the fixed age structure obtained from the 2006 census in which 47.5% were under 15 years of age, 49.6% were aged 15-64 and 2.9% were 65 and above. The fixed age structure undermines planning for age-specific groups in the population. Fertility rates have changed substantially from the intercensal experience, and so have mortality and migration trends. Thus, there is need for population projections that incorporate these changes.