## Data Collection

## Data Collection

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## Methodology

General methodology is outlined in the National Population Projections data collection.

**Main changes to assumptions since the previous 2016-base projections**

Deriving the projections involved a review of all projection assumptions. The main changes from the previous 2016-base projections relate to the base population, migration, and fertility assumptions.

The **base population** at 30 June 2020 of 5.094 million is 89,200 (1.8 percent) higher than the 2016-base median projection for 2020. This is mainly reflecting the changes between 2013-base and 2018-base estimated resident population.

See Estimated resident population (2018-base): At 30 June 2018 for more information about the base population.

The median **annual net migration gain** is assumed to be 25,000 in 2023 and beyond, an increase from the long term level assumed in the 2016-base projections of 15,000. In the short term, the median net migration assumptions are : 10,000 in the June year 2021, increasing to 20,000 in 2022. Simulations of net migration are produced using an ARIMA(1,0,1) model, the same model used in the 2011-base and 2016-base projections, rather than the ARIMA(0,1,2) model used in the 2014-base projections.

The median period total fertility rate (TFR) is assumed to be 1.65 births per woman throughout the projection period, a decrease from the long term level assumed in the 2016-base projections of 1.85 births per woman. In the 32 years from 1977 to 2009, the period TFR was generally in the range of 1.9–2.2 births per woman. From 2010, the TFR has dropped from 2.18 to 1.66 in 2020.

**Stochastic (probabilistic) population projections**

Stochastic (probabilistic) population projections are produced to give estimates of uncertainty, although these estimates are themselves uncertain. The stochastic population projections are produced by combining 2,000 simulations of the assumptions. 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 for the 2020-base projections.

At the time of release, the median projection (50th percentile) indicates an estimated 50 percent chance that the actual value will be lower, and a 50 percent chance that the actual value will be higher, than this percentile.

**The median projection for 2020-base projections assumes:**

- the total fertility rate stays 1.65 births per woman throughout the projection period
- period life expectancy at birth increases to 87.2 years for males and 89.9 years for females in 2070
- annual net migration of 10,000 in 2021, 20,000 in 2022, 25,000 in 2023 and beyond.

**'What if?' scenarios**

Five 'what if?' scenarios have been produced to illustrate what happens when different specific levels of fertility, mortality, and migration assumptions are combined.

**Very high fertility** assumes:

- the total fertility rate increases to 2.3 births per woman in 2040 and beyond
- period life expectancy at birth increases to 87.2 years for males and 89.9 years for females in 2070
- annual net migration of 10,000 in 2021, 20,000 in 2022, 25,000 in 2023 and beyond.

**Very low mortality** assumes:

- the total fertility rate stays 1.65 births per woman throughout the projection period
- period life expectancy at birth increases to 96.0 years for both males and females in 2073
- annual net migration of 10,000 in 2021, 20,000 in 2022, 25,000 in 2023 and beyond.

**No migration** assumes:

- the total fertility rate stays 1.65 births per woman throughout the projection period
- period life expectancy at birth increases to 87.2 years for males and 89.9 years for females in 2070
- no external migration from 2021 onwards (ie a 'closed' population).

**Cyclic migration** assumes:

- the total fertility rate stays 1.65 births per woman throughout the projection period
- period life expectancy at birth increases to 87.2 years for males and 89.9 years for females in 2070
- annual net migration fluctuates specifically between -5,000 and +60,000 on a 10-year cycle, with an average of 25,000.

**Very high migration** assumes:

- the total fertility rate stays 1.65 births per woman throughout the projection period
- period life expectancy at birth increases to 87.2 years for males and 89.9 years for females in 2070
- annual net migration of 20,000 in 2021, 40,000 in 2022, 50,000 in 2023 and beyond.

See National population projections: 2020(bae)–2073 for more details on projection assumptions.

**Accuracy of projections**

The accuracy of these projections is unknown at the time of release. While the assumptions are formulated from an assessment of short-term and long-term demographic trends, there is no certainty that any of the assumptions will be realised. The projections do not take into account non-demographic factors (eg war, catastrophes, major government and business decisions) which may invalidate the projections.

See How accurate are population estimates and projections? An evaluation of Statistics New Zealand population estimates and projections, 1996–2013 for an evaluation of previous Statistics NZ national and subnational population estimates and projections.

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