This is the female resident population projected to be aged 21 in the reference year based on the 2018 mid year population estimates.
Population projections are an indication of the future trends in population over the next 25 years. They are trend-based projections, which means assumptions for future levels of births, deaths and migration are based on observed levels mainly over the previous 5 years. They show what the population will be if recent trends continue. They are not forecasts and do not attempt to predict the impact that future government or local policies, changing economic circumstances or other factors might have on demographic behaviour.
The projected resident population of an area includes all people who usually live there, whatever their nationality. People moving into or out of the country are only included in the resident population if their total stay in that area is for 12 months or more, thus visitors and short-term migrants are not included. Armed forces stationed abroad are not included, but armed forces stationed within an area are included. Students are taken to be resident at their term-time address.
Projections become increasingly uncertain the further they are carried forward due to the inherent uncertainty of demographic behaviour. This is particularly so for smaller geographical areas and detailed age and sex breakdowns.