New Zealand Cohort Life Tables

High-level description of data collection and processing methods used.

This series adopts a component-based approach to construct the demographic history of each birth cohort.

The components of population change

births, deaths and migration – are used to determine the initial birth cohort size and changes at each subsequent year and age.

Particular attention is given to quality issues pertaining to data coverage and the levels of specification available, which impact on the robustness of cohort life tables.

The cohort component method was used to derive the population projections. By this method, the base population is projected forward by calculating the effects of deaths and migration within each age-sex group according to specified mortality and migration assumptions. New birth cohorts are generated by applying specified fertility assumptions to the female population of childbearing age.

Users of the cohort life tables should note the following:

• The derivation of the cohort life tables required the application of various data estimations and assumptions. The mortality history of each birth cohort has been built up from births, deaths, and migration data which vary in coverage and completeness over time. Care should be exercised when interpreting the cohort mortality tables to avoid over-precise comparisons, especially between adjacent birth cohorts and at the oldest ages where the cohort populations are small. The cohort mortality results are subject to ongoing revision as additional births, deaths, and migration data becomes available.
• The births and deaths data presented in the cohort mortality study will differ from that used in other fertility and mortality statistics .

The study is based on all (live) births and deaths occurring in New Zealand according to the year they occurred, in addition to war deaths occurring outside of New Zealand.

The population denominators for the study were derived solely from the components of population change (estimated births, deaths, and external migration). In contrast, other fertility and mortality statistics (eg annual fertility rates, period life tables) are based on (live) births and deaths occurring in New Zealand according to the year they were registered. Since 1991, the published statistics have also excluded events to people visiting from overseas.

The population denominators for those statistics are derived from census counts.

• The projection of cohort death rates at ages above 74 years is designed to extend the mortality analysis for cohorts who have not yet completed their life span. By this extension, 1941 is the latest birth cohort for whom life expectancy can be calculated. In presenting the results of cohort life tables, projected life table data are distinguished from historical data by the use of italics in tables and dashed lines in figures.

Data source

We have built up the mortality history of each birth cohort from births, deaths, and migration data, which vary in coverage and completeness over time. Deriving the cohort life tables required us to apply various data estimations and assumptions.

Births, Deaths, and Marriages (a division of the Department of Internal Affairs) supplies us with birth and death registrations. Under the provisions of the Births, Deaths, Marriages, and Relationships Registration Act 1995, every birth and death occurring in New Zealand must be registered with Births, Deaths, and Marriages.

The New Zealand Customs Service supplies us with migration data. This data is received monthly and processed by our Population Statistics Unit.

Consistency with other periods or datasets

The births and deaths data presented in these cohort life tables will differ from that used in other fertility and mortality statistics. The tables are based on all (live) births and deaths in New Zealand according to the year they occurred, in addition to war deaths outside New Zealand.

We derived the population denominators for the cohort life tables solely from the components of population change (estimated births, deaths, and external migration).

In contrast, other fertility and mortality statistics (eg annual fertility rates, period life tables) are based on (live) births and deaths in New Zealand according to the year they were registered. Since 1991, they excluded births and deaths of people visiting from overseas. We derive the population denominators for these statistics (eg annual fertility rates) from census counts.