About national population projections
National population projections give an indication of the future population usually living in New Zealand based on different combinations of fertility, mortality, and migration assumptions.
These projections are not predictions. The projections should be used as an indication of the overall trend, rather than as exact forecasts. The projections are updated every 2–3 years to maintain their relevance and usefulness, by incorporating new information about demographic trends and developments in methods.
More definitions
Assumption: statement about a future course of behaviour (eg fertility, mortality, migration) from which projections of the population are derived.
Baby boomer: someone born in the years 1946–65, although the definition of the baby boom period varies between sources and between countries.
Base population: the starting population for the projections.
Cohort: a group of people sharing a common experience. For example, the 1900 birth cohort refers to people born in the year 1900.
De facto population concept: a statistical basis for a population in terms of those present in a given area at a given time. For example, the 'estimated de facto population' of New Zealand is an estimate of all people present in New Zealand at a given date, including visitors from overseas, but excluding New Zealand residents who are temporarily overseas.
Deterministic projection: a single projection from a given set of assumptions (eg about fertility, mortality, migration).
Estimated resident population: an estimate of all people who usually live in New Zealand at a given date. It includes:
- all residents present in New Zealand and counted by the census (census usually resident population count)
- residents who are temporarily overseas (who are not included in the census)
- an adjustment for residents missed or counted more than once by the census (net census undercount).
It excludes visitors from overseas.
Fertility: the demographic process relating to births, often summarised by birth rates and fertility rates. Fertility should not be confused with fecundity, which is the biological capacity of a population to bear children.
Life expectancy (cohort): the average length of life remaining at a given age, experienced by people born in the same year. For example, life expectancy at birth for people born in 1900 is based on death rates experienced by those people at each age throughout their life.
Life expectancy (period): the average length of life remaining at a given age, assuming people experience the age-specific death rates of a given period from the given age onwards. For example, life expectancy at birth for the period 2005–07 is based on death rates in that period, and takes no account of changes in death rates after that period.
Median age: half the population is younger, and half the population is older, than this age.
Median projection: the 50th percentile, which indicates an estimated 50 percent chance that the actual result will be lower, and a 50 percent chance that the actual result will be higher, than this percentile.
Mortality: the demographic process relating to deaths, often summarised by death rates, survival rates, and life expectancy.
Percentile: indicates the distribution of values (such as projection results or assumptions). For example, the 25th percentile indicates an estimated 25 percent chance that the actual result will be lower, and a 75 percent chance that the actual result will be higher, than this percentile.
Percentiles are non-additive except the 50th percentile (median). For example, percentiles for the population aged 15–39 and 40–64 years cannot be added together to give the equivalent percentile for the population aged 15–64 years.
Shading in graphs indicates the chance that actual results will fall within a certain range. Different shading is used to distinguish different ranges.
Projection: indication of the future characteristics of a population based on an assessment of past trends and assumptions about the future course of demographic behaviour (eg fertility, mortality, migration).
Replacement fertility: the average number of live births that women need to have for a population to replace itself in the long term, without migration. This equates to a total fertility rate of about 2.1 births per woman, which allows for the sex ratio at birth (roughly 105 males born for every 100 females) and for mortality of females between birth and before they have children of their own.
Resident population concept: a statistical basis for a population in terms of those who usually live in a given area at a given time. For example, the 'estimated resident population' of New Zealand is an estimate of all people who usually live in New Zealand at a given date, including New Zealand residents who are temporarily overseas, but excluding visitors from overseas.
Stochastic (probabilistic) projection: a projection which varies randomly according to the probability distributions of the assumptions (eg about fertility, mortality, migration).
Total fertility rate (cohort): the average number of live births that women born in the same year have had during their life. For example, the total fertility rate for women born in 1960 is based on fertility rates experienced by those women at each age throughout their life.
Total fertility rate (period): the average number of live births that women would have during their life if they experienced the age-specific fertility rates of a given period. For example, the total fertility rate for the year 2011 is based on fertility rates in that year, and takes no account of changes in fertility rates after that year.