## Data Collection

## Data Collection

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## Methodology

**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.

The stochastic approach used in the national population projections since the 2011-base projections involves creating 2,000 simulations for the base population, births, deaths, and net migration, and then combining these using the cohort component method.

These simulations can be summarised by percentiles, which indicate the probability that the actual result is lower than the percentile. For example, the 25th percentile indicates an estimated 25 percent chance that the actual value will be lower, and a 75 percent chance that the actual result will be higher, than this percentile.

Nine alternative percentiles of probability distribution (2.5th, 5th, 10th, 25th, 50th, 75th, 90th, 95th, and 97.5th percentiles) are available in NZ.Stat.

**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 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 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

**2020-base to 2073**

The 2020-base national population projections (released December 2020) have as a base the estimated resident population (ERP) of New Zealand at 30 June 2020, and cover the period to 2073 at one-year intervals. They superseded by the 2016-base national population projections (released October 2016).

Detailed information on the 2020-base population, assumptions and 'what if' scenarios used can be found here National Population Projections 2020-base

**2016-base to 2068**

The 2016-base national population projections (released October 2016) have as a base the provisional estimated resident population (ERP) of New Zealand at 30 June 2016, and cover the period to 2068 at one-year intervals. They supersede the 2014-base national population projections (released November 2014).

Detailed information on the base population, assumptions and 'what if' scenarios can be found here: National Population Projections 2016-base

**2014-base to 2068**

The 2014-base national population projections have been superseded by the 2011-base national population projections (released September 2012).

Detailed information on the 2014 base population, assumptions and 'what if' scenarios used can be found here National Population Projections 2014-base

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