National Population Projections (series)

Method The 'cohort component' method has been used to derive the population projections. Using this method, the base population is projected forward by calculating the effect of deaths and migration within each age-sex group (or cohort) according to the specified mortality and migration assumptions. New birth cohorts are added to the population by applying the specified fertility assumptions to the female population of childbearing age. Stochastic Approach Since the 2011-base projections, a stochastic approach has been used. This involves creating 2,000 simulations for the base population, births, deaths, and net migration. These simulations are combined using the cohort component method and summarised by percentiles (e.g., 25th, 75th), which indicate the probability distribution of outcomes. For example, the 25th percentile indicates an estimated 25 percent chance that the actual value will be lower, and a 75 percent chance it will be higher. Nine alternative percentiles of probability distribution (2.5th, 5th, 10th, 25th, 50th, 75th, 90th, 95th, and 97.5th percentiles) are available in Aotearoa Data Explorer. Projection Assumptions Projection assumptions are formulated after analysis of short-term and long-term demographic trends, patterns and trends observed in other countries, government policy, information provided by local planners, and other relevant information. Assumptions for national projections are derived for each single-year of age to produce projections at one-year intervals. The following describes how assumptions are applied for national projections. Fertility Projected (live) births are derived by applying age-specific fertility rates to the mean female population of childbearing age. The mean female population for each age is derived by averaging the population at the start and end of each year. The sum of the number of births derived for each age of mother gives the projected number of births for each year. The female age-specific fertility rates for each year of the projection period represent the number of births to females of each age in each year. The set of age-specific fertility rates for each year is typically summarised by the period total fertility rate. For all population projections, a sex ratio at birth of 105.5 males per 100 females is assumed, based on the historical annual average of the total population. The fertility assumptions should not be used as a precise measure of fertility or of fertility differentials between groups. It is important to note that the objective of population projections is not to specifically measure or project the fertility of the population. For projection purposes it is more important to have a realistic yet tractable model for projecting fertility trends (and birth numbers) into the future. Mortality Mortality assumptions are formulated in terms of survival rates. This is because in the projection model the base population is survived forward each year. The projected number of deaths is calculated indirectly. Survival rates are applied to births and single years of age. There are different survival rates for each age of life and for males and females. The male and female age-specific survival rates for each year of the projection period represent the proportion of people at each age-sex who will survive for another year. In general, survival rates are highest at ages 5–11 years and then decrease with increasing age. The set of age-sex-specific survival rates for each year is typically summarised by male and female period life expectancies at birth. Annual survival rates are applied separately to the population at the start of each year, births and migrants. The mortality assumptions should not be used as a precise measure of mortality or of mortality differentials between groups. It is important to note that the objective of population projections is not to specifically measure or project the life expectancy of the population. For projection purposes it is more important to have a realistic yet tractable model for projecting mortality trends (and death numbers) into the future. Migration Migration assumptions are formulated in terms of a net migration level and an age-sex net migration pattern for each year of the projection period. Where practical, both the level and age-sex pattern are derived from a detailed analysis of net migration, including: external migration data (from passenger cards): arrivals and departures by country of citizenship New Zealand citizen arrivals and departures by country of source/destination Immigration New Zealand data: residence applications and approvals student and work visas Ministry of Education figures on students from overseas in formal education. Application For each projection year the assumptions are applied thus: survive the population alive at the start of the year derive the number of migrants by age and sex survive the migrants derive the number of births (by applying fertility rates to mean of female population at start and end of the year) survive the births. For ethnic population projections, additional assumptions are applied thus: survive the population alive at the start of the year derive the number of migrants by age and sex survive the migrants adjust the population to account for inter-ethnic mobility (by applying rates to the population at the start of the year) derive the number of births (by applying fertility rates to mean of female population at start and end of the year, and applying paternity rates to mean of male population at start and end of the year) adjust the births to account for births to parent(s) of that ethnicity that are not registered as children of that ethnicity (by applying net rate to projected births) survive the births. Population (at start of the year) For the population already present in the country at the start of the year, the number alive at the end of the year is calculated as: Survived Population = Population x Survival Rate There are different survival rates for each age of life, from age 0 to age 100+ years, and for males and females. Birth Births occur throughout a year and not all children born during a year will be alive at the end of the year. For this reason a survival rate for births is needed: Survived Births = Births x (1 + Survival Rate) / 2 This formula is derived by taking the average of: assuming all births occurred on 1 July (the first day of the year) of each year, and assuming all births occurred on 30 June (the last day of the year) of each year. If all births occurred on 1 July then the births have to be survived over a whole year. If all births occurred on 30 June then there would be no deaths and hence there is no need to apply a survival rate. The average of these two is taken because births occur throughout the year. Migrants A similar mathematical formula is used to survive the migrants for each age: Survived Migrants = Migrants x (1 + Survival Rate) / 2 The assumption is made that either all migration occurs on the first day of the year or all migration occurs on the last day of the year. The average is then taken because migration occurs throughout the year. Projection Series Base Year Projection Period Details 2024-base 2024–2078 See assumptions 2022-base 2022–2073 See assumptions 2020-base 2020–2073 See assumptions 2016-base 2016–2068 See assumptions 2014-base 2014–2068 See assumptions en-NZ