Annex A: Notes on Population Data
The Population Projections for Scottish Areas are based on the latest mid-2016 population estimates and provide an indication of the future population size and age structure of Scottish areas based on a set of assumptions about future fertility, mortality and migration:
- Fertility is taken to mean the total number of children a woman would have, on average at the end of her child bearing years. The long-term fertility rate for Scotland is assumed to be 1.65. Past trends in the number of births in each area is compared with the national trends. This is used to calculate a differential rate that is then applied to the Scotland level fertility rates to calculate the future number of births in each area.
- At Scotland level, future improvements in mortality rates are based on the trend observed in the period 1961 to 2015. Based on these rates, the expectations of life at birth for Scotland are projected to increase from 77.1 years for those born around 2015 to 79.4 years in 2026 for males, and from 81.2 years for those born around 2015 to 82.7 years in 2026 for females. The number of deaths in each area is compared with the national trends. For deaths this is done separately by sex for three age groups; 0-59, 60-79 and 80 and over. This is used to calculate a differential rate that is then applied to the Scotland level mortality rates to calculate the future number of deaths in each area.
- Three types of migration are modelled separately by the projections; within Scotland migration, migration between Scottish areas and the rest of the UK and migration between Scottish areas and overseas.
- For within Scotland migration and rest of UK migration, trends in migration over the previous five years is used to create a rate that is then applied to the projected population for each area to calculate the number of in and out migrants.
- Overseas migration is projected separately It includes migration by asylum seekers and refugees, both of which are modelled separately from other international migrants. It is assumed in the national population projections (NPP) that international migration will move from levels seen in the base year to a long-term trend several years later. The time taken to reach the long-term trend is known as the run-in period. In the 2016-based population projections, the run-in period was seven years, with the long-term trend beginning in 2023.
- For each area, the projected population for each year is calculated by removing any special populations (such as prisoners and armed forces) from the previous year's population and then ageing on the remaining population. Local fertility and mortality rates are then applied to calculate the number of projected births and deaths. Rates to calculate migration within Scotland and with the rest of the UK are also applied, before migrants to and from overseas are added and subtracted from the population. Finally, any special populations are added back in to find the final projected population. This is then repeated for each year of the projection.
- The assumptions are based on past trends and do not take account of any future changes that may occur as a result of policy initiatives but may reflect the past impact of policy and economic changes.
- Variant projections using alternative plausible assumptions are also produced. The seven variants included are: high fertility, low fertility, high life expectancy, low life expectancy, high migration, zero outwith Scotland migration.