National population projections by age and gender are produced every two years for the UK and each of the UK constituent countries by the Office for National Statistics (ONS) on behalf of the National Statistician and the Registrars General of Scotland and Northern Ireland. Principal Projections are produced based on long-term assumptions about future fertility, mortality and migration levels, with variant projections being produced from various alternative assumptions – usually in the form of high/low variants and their combinations. National population projections provide an estimate of the future size and age structure of the population of Northern Ireland. Population projections are widely used in policy development in areas such as housing, healthcare and education.