New Zealand Period Life Tables

High-level description of data collection and processing methods used in life tables from the release of the 2012–14 life tables in 2015.

This part details the data and methods used to derive the complete life tables .

Data

The data used to construct the 2012–14 life tables comprises:

• Deaths registered in NZ of people resident in NZ in the December years 2005–07 and 2012–14, respectively, by date of death, date of birth, sex, ethnicity and regional council of usual residence.
• The estimated resident population of NZ at 30 June 2006 and 2013, by single year of age, sex, ethnicity and regional council.

Note: For statistical modelling purposes data for both periods 2005–07 and 2012–14 are used.

Deaths numerator

The life tables are compiled from deaths registered, rather than deaths occurring, in each respective three-year period. Most death statistics refer to registrations rather than occurrences for a given time period, because there is generally a time lag between when the death occurred and when it is registered. For this reason, the number of death registrations can be determined before the number of death occurrences for a given time period. For periods of a year or more, the difference between registrations and occurrences is generally small, so death statistics referring to registrations are suitable for most purposes.

Population denominator (exposed-to-risk population)

The 2012–14 life table calculations use as the population denominator the estimated resident population at 30 June 2013, the mid-point of the period. The total population estimate of 4,442,100 included 692,300 people who identified with the Māori ethnic group, and 344,400 who identified with Pacific ethnicities. By subtraction, the non-Māori population was 3,749,800.

A demographic analysis using cumulated counts of deaths from the highest ages was used to compare counts of the population previously alive (‘extinct generation method’) with the estimated resident population for each population group at 30 June for both 2006 and 2013. As a result of this analysis, ages 100 years or over are represented by one open age group in the life tables, and life table calculations use population data generated by the ‘extinct generation method’ for the oldest ages.

Ethnicity non-response in deaths data

Individual death records with missing ethnicity responses are assigned combinations of ethnicity indicators based on a statistical multiple imputation approach. Death records with observed ethnicity response for the two time periods (2012–14 and 2005–07), and the population exposure at the period mid point (at 30 June 2013 and 30 June 2006) are the inputs to the statistical model. The model uses 10-year age groups, sex, regional council, and the two time periods.

The statistical model fitted to the data generates estimates of rates for identifying with an ethnic group or combination of ethnicities. Values for the ethnic indicators in death records with missing response were generated using a probability distribution given the observed rates for identifying with an ethnic indicator estimated by the model.

Derived rates

The life tables use age-sex-specific death rates calculated using the average number of deaths over three years. Doing this reduces the effect of year-to-year statistical variations, particularly at younger ages where there is a small number of deaths or no registrations, and at very old ages where the population-at-risk is small. The observed age-sex-specific death rate for the period is calculated as the ratio of the average number of deaths between ages x and x+1 (x = 0, 1, 2, …, 100+) and the estimated population at the period mid-point (at 30 June 2013).

The observed death rate is likely to show some random variation at ages with small death counts (or missing counts) or population-at-risk, and observed rates will fluctuate considerably from one period to another for these ages. Implementing a hierarchical statistical model provides a coherent solution for deriving estimates of the age-sex-specific death rate. Data for the two periods, 2012–14 and 2005–07, were fitted to the model, and model estimates of age-sex-period-specific death rates (including uncertainty measures) were used for deriving the death probabilities.

The conversion of age-sex-period-specific death rates to probabilities of dying at each age x to x+1 includes the derivation of average number of person-years lived in the period by those dying in the same period (Preston et al, 2001). For single year cohorts aged 1, …, 100+ years, it is assumed that, on average, people dying in the period do so half-way through this period (0.5). Infant mortality observed in the youngest cohort (age 0 years) will be more concentrated at the earlier stages of infancy. Based on date of birth information for deaths in their first year of life the average number of person-years lived for this cohort was estimated at 0.1.

Measures of uncertainty

The statistical approach for estimating period death rates offers an explicit way of including uncertainty in the data and the parameters of the chosen model. This includes integrating a measure of uncertainty derived at the imputation step for assigning ethnic indicators to records with missing ethnicity responses (described above). The 50th percentile (median), 2.5th percentile and 97.5th percentile estimates of the death rate and other life table variables are available at the detailed demographic level. Under the model, 95 percent credible intervals include the true underlying value 95 percent of the time.

Age contribution to longevity differences

We can determine the contribution that each age group has made to longevity differences, between periods or between populations (eg, Māori and non-Māori). Ages do not contribute equally to life expectancy at birth, with the youngest ages contributing relatively more. The comparison involves calculating and comparing hypothetical or temporary life expectancies at each age. Hypothetical life expectancy is the average number of years that a group of people will live from age x to x + i years (where i is the age interval). For more information see Arriaga (1984).

Ethnic concept

The ethnic concept used in these period life tables is the ethnic group or groups that people identify with or feel they belong to. Ethnicity is self-perceived and people can belong to more than one ethnic group. For example, people can identify with Māori ethnicity even though they may not be descended from a Māori ancestor. Conversely, people may choose to not identify with the Māori ethnicity even though they are descended from a Māori ancestor. Ethnicity is not the same as birthplace.

For births and deaths registrations, ethnicity is identified by the person completing the registration form. For deaths, this person is most likely to be the funeral director (on the advice of a family member).

Note: New Zealand Period Life Tables: 2012–14 supersedes the 2010–12 results, which we released in 2013 to cover the interim period. We’ve removed the 2010–12 series from Infoshare.

High-level description of data collection and processing methods used prior to the release of the 2012–14 life tables in 2015.

This part details the data and methods used to derive the 1995–97 (revised) to 2010–12 complete life tables .

Data

The data used to construct the 1995–97 (revised) to 2010–12 complete life tables were:

• deaths registered in New Zealand of people resident in New Zealand in the December years 1995–97, 2000–02, 2005–07 and 2010–12, by single-year of age, sex and total
• live births registered in New Zealand to mothers resident in New Zealand in the December years 1994–97, 1999–2002, 2004–07 and 2009–12 by sex, total, and ethnicity
• the estimated resident population of New Zealand at 30 June 1996, 30 June 2001, and 30 June 2006 by single-year of age, sex total, and ethnicity.

Deaths numerator

The life tables are compiled from deaths registered, rather than deaths occurring, in each respective three-year period. Most death statistics refer to registrations rather than occurrences for a given time period, because there is generally a time lag between when the death occurred and when it is registered. For this reason, the number of death registrations can be determined before the number of death occurrences for a given time period. For periods of a year or more, the difference between registrations and occurrences is generally small, so death statistics referring to registrations are suitable for most purposes.

An allowance for ethnic non-response among deaths was also made. There was no response to the ethnicity question for about 2.4 percent of deaths in 1996–97, 3.5 percent of deaths in 2000–02, 4.9 percent of deaths in 2005–07 and 0.7 percent of deaths in 2010–12. Because deaths in the first year of life are skewed towards the early part of this age, infant death rates were calculated from more detailed data. This involved the division of the first year of life into more detailed ages.

Population denominator (exposed-to-risk population)

Usually the mean population for a period is used as the denominator to calculate death rates. However, mean population estimates are not available for all ethnic populations. To ensure consistency of method among all population subgroups, the estimated resident population at 30 June (the midpoint) was used. The impact of using ‘midpoint’ rather than ‘mean’ population estimates is generally insignificant.

The estimated resident populations at 30 June 1996, 30 June 2001 and 30 June 2006 were based on the census usually resident population counts at 5 March 1996, 6 March 2001 and 7 March 2006, respectively, and adjusted for:

• non-response to the census ethnicity question - net census undercount
• residents temporarily overseas on census night
• births, deaths and net migration between census night and 30 June of the census year
• reconciliation with demographic estimates at ages 0–4 years.

Quarterly national population estimates from September 2006 onwards were derived by updating the estimated resident population at 30 June 2006 for births, deaths, and net migration during the ensuing period. Māori population estimates were also updated for inter-ethnic mobility during the ensuing period. Non-Māori population estimates are derived by subtracting Māori from the total population estimates.

The 1996 Census asked people to "Tick as many circles as you need to show which ethnic group(s) you belong to". The census usually resident population count of 3,618,303 included 523,371 who identified with the Māori ethnicity and 151,713 who gave no specific ethnic response. The 1995–97 life tables use as a population denominator, the estimated resident population of each ethnicity of New Zealand at 30 June 1996. New Zealand's estimated resident population of 3,732,000 included 573,000 who identified with the Māori ethnicity.

The 2001 Census asked people "Which ethnic group do you belong to? Mark the space or spaces which apply to you". The census usually resident population count of 3,737,277 included 526,281 who identified with the Māori ethnicity and 150,546 who gave no specific ethnic response. The 2000–02 life tables use as a population denominator, the estimated resident population of each ethnicity of New Zealand at 30 June 2001. New Zealand's estimated resident population of 3,880,000 included 586,000 who identified with the Māori ethnicity.

The 2006 Census asked people "Which ethnic group do you belong to? Mark the space or spaces which apply to you". The census usually resident population count of 4,027,947 included 565,329 who identified with the Māori ethnicity and 167,784 who gave no specific ethnic response. The 2005–07 life tables use as a population denominator, the estimated resident population of each ethnicity of New Zealand at 30 June 2006. New Zealand's estimated resident population of 4,185,000 included 624,000 who identified with the Māori ethnicity.

Because of changes to the census ethnicity question between 1996 and 2001, the 1996, 2001, and 2006 population estimates for ethnic groups are not necessarily comparable. Nevertheless, the derived mortality measures presented here are considered to give a statistically satisfactory description of Māori and non-Māori mortality experience during the 1995–97, 2000–02, 2005–07 and 2010–12 periods. Note: Both the 2005–07 and 2010–12 interim life tables use population estimates on 2006 base.

Note: All population estimates used in the 1995–97, 2000–02, 2005–07 and 2010–12 life tables have been derived using the same methodology. In addition, the ethnicity question used in the 1996 Census is the same as that used in birth and death registration forms from September 1995.

The use of population estimates based on the 1996 Census also allows the adjustment ratios presented in Ajwani (2003) to be incorporated.

For more information about the estimated resident population, refer to "Information about the population estimates".

Derived rates

The period life tables were based on deaths averaged over three years. This is designed to reduce the impact of year-to-year statistical variations, particularly at younger ages where there are few deaths and at very old ages where there are few people. At some ages the New Zealand data does not enable death rates to be reliably estimated. For most ages above 90 years, death rates of the total New Zealand population have been modelled on the mortality trends of other countries such as Australia, Canada, Japan, United Kingdom and United States. At the older ages it is assumed that the rate of change in probability of dying within a year(qx) decreases linearly with increasing age. For the Māori and non-Māori populations, death rates have also been modelled at ages in the early 80s and late 80s, respectively .

Because deaths in the first year of life are skewed towards the early part of this age, infant death rates were calculated from more detailed data. This involved the division of the first year of life into more detailed ages. The report New Zealand Life Tables: 2005–07 describes the method in more detail. Death rates at age 0 and 1 years also use estimated birth occurrences rather than birth registrations during 2010–12. This allows for births that have not been registered within that period.

There are some small observed numerator-denominator ethnic differences since 1995 in deaths as compared to 1996, 2001 and 2006 census data.

For the 2000–02, 2005–07 and 2010–12 life tables, these estimated differences are not significant enough to reliably adjust death numbers by age, sex and ethnicity. For the 1995–97 life tables, the smooth adjustment factors presented in Ajwani (2003) have been applied to Māori deaths by age to allow for under-reporting of Māori deaths (relative to the Māori population). The adjustment factors range from 0.94 (ie a deflation in deaths) at age 0 years to 1.14 at ages 20–24 years.

For the non-Māori period life tables, corresponding adjustments have been applied to non-Māori deaths by age. These adjustment factors affect Māori life expectancy at birth by about 0.7 years and non-Māori life expectancy at birth by about 0.1 years.

The 1995–97 period life tables for the total New Zealand, Māori and non-Māori populations published in July 1998 have therefore been revised. For all populations the revision also incorporates updated population estimates (at 30 June 1996) and a revised method of estimating death rates at the oldest ages.

The construction of each complete period life table involved two stages.

First, the central death rates (mx) were calculated for each age (x), except the first year of life, and were then smoothed to eliminate any apparent irregularities.

Second, the smoothed rates were used to calculate a set of age-specific probabilities of death (qx), which were then used to derive other life table functions.

The derivation of the mortality rate in the first year of life differed from all other ages and required special formulae.

More detailed methodology is available on request :

• derivation of the mortality rate in the first year of life;
• central death rates (mx) for age 1 and over;
• supplementary functions for five-year age groups