Fertility | Internal migration | International travel and migration | Lifecycle | Life expectancy | Life table | Migration | Momentum | Mortality | Natural increase | Net migration | Net reproduction rate
Fertility
There are many measures of fertility referring to the level of reproduction of a population, based on the number of live births that occur, have occurred or are expected to occur.
When dealing with vitals data, fertility is normally measured in terms of women of childbearing age. When stating measures the definition of "childbearing age" should always be given – for example, as 15-44 years or 15-49 years. Using a specified age range avoids misleading information although a small number of births to women outside this age range can and do occur.
In the context of the census of population and dwellings, fertility refers to data derived from the question on the number of children ever born alive to each female, resident in New Zealand and aged 15 years or over, at the time of data collection.
Internal migration
The movement of people within New Zealand.
International travel and migration
The movement of people to and from New Zealand. International travel and migration statistics are compiled from individual migration forms filled in by passengers arriving in and departing from New Zealand, and forwarded to Statistics New Zealand by the Customs Department. For analytical purposes, people generally fall into one of three classes:
- overseas travellers in New Zealand for a short-term (less than 12 months) stay
- New Zealand residents away for a short-term (less than 12 months) absence
- permanent and long-term international migrants.
Permanent and long-term arrivals include overseas migrants who arrive in New Zealand intending to stay for a period of 12 months or more (or permanently), plus New Zealand residents returning after an absence of 12 months or more.
Permanent and long-term departures include New Zealand residents departing for an intended period of 12 months or more (or permanently), plus overseas visitors departing from New Zealand after a stay of 12 months or more.
Lifecyle
A division of the lifespan into socially relevant units of time, generally containing stages relating to birth and the early years, childhood, transition to adulthood, young adulthood, middle age, the young old, and the old old. The precise number of life cycle stages and their age ranges reflect prevailing social attitudes and are defined appropriately for the context of the analysis.
Life expectancy
The average length of life remaining at a given age. As derived from a period life table, it assumes that a person experiences the age-specific mortality rates of a given period from the given age onwards.
For example, life expectancy at birth refers to the average length of life a newborn baby may expect assuming they experience the age-specific mortality rates of a given period throughout their life. It represents the average longevity of the whole population and does not necessarily reflect the longevity of an individual.
See also Mortality (in this glossary, below).
Life table
A tabular numerical representation of mortality and survivorship of a cohort of births at each age of life. It comprises an array of measures, including probabilities of death, probabilities of survival, and life expectancies at various ages.
Current, period or cross-sectional life tables are based on current mortality rates. These tables assume that as a cohort passes through life it experiences a given pattern of age-specific mortality rates which do not change from year to year. Although usually based on death rates from a real population during a particular period of time, these tables are a hypothetical model of mortality as they do not describe the real mortality that characterises a cohort as it ages.
A complete life table presents life table functions for each single year of age, while an abridged life table presents life table functions for age groups.
Migration
The movement of people from one area to another. When the movement is between countries it is called international travel and migration; when it is within a country it is called internal migration.
Momentum (also occasionally referred to as pipeline effects)
Population momentum is the potential for growth (or decline) inherent within an age structure.
In general, a youthful age structure contains momentum for growth, while an older age structure may contain momentum for decline if the population has not reached a point of stability.
Momentum effects explain why the population continues to increase after the total fertility rate has fallen. This is because many women in the large cohorts born during periods of high fertility will have children themselves within 15-40 years – they may each have fewer children on average than their older peers but the number of women having children is much larger, thus creating a potential for growth.
The New Zealand population in the 1970s and 1980s had an inbuilt momentum for growth due to the ongoing effects of the baby boom period.
In the future, with continued low fertility, New Zealand may experience a momentum of decline if the number of deaths exceeds the number of births (excluding migration effects). Momentum or pipeline effects can lead to short-term rapid changes in demand for services due to changing numbers in the relevant age groups.
Mortality
Death, expressed either in terms of the number of people dying or as a proportion of a specified population dying in a specified period.
Natural increase
The difference between the number of live births and the number of deaths. When births exceed deaths, natural increase is positive. When deaths exceed births, natural increase is negative.
Net migration
The difference between arrivals into and departures from a region or country. Net permanent and long-term migration contributes to the population growth rate (along with natural increase).
Net reproduction rate
The average number of daughters that a woman would have during her life if she experienced the age-specific fertility and mortality rates of that year. When the Net Reproduction Rate is one, each woman in the population would, on average, have one daughter.