Overall population movements, such as those mentioned above, cover a considerable amount of variation across the underlying characteristics of the population. For example, immigration and emigration propensities are typically higher amongst individuals in their early to mid-20s, western immigrants have a higher tendency to emigrate than non-western immigrants and women have lower mortality rates than men in the same age groups.
There is a considerable uncertainty associated with the forecast of these population movements. This is obvious in the estimates of the number of future immigrants. This is attributable to the fact that immigration is largely controlled by policy and subject to developments in international conditions, both of which are hard to estimate and forecast. The abnormal numbers for immigration in 2015 are an example of the difficulties surrounding migration forecasting. The number of births also varies from year to year and highlights another source of challenge when estimating population. Developments in mortality rates have been relatively stable for a number of years, however, it is nevertheless possible to underestimate the actual mortality, especially in the case of extraordinary circumstances such as disease epidemics and extreme climatic conditions.
The forecast provides estimates for the difference between people inside and outside of working age, a factor crucial to assessing the fiscal challenges that future generations can expect to face. The overall pattern observed in the forecast is one that shows a decreasing proportion of the population within the working age.