Life tables
Life tables are one of the basic demographic tools for analysing mortality. They are a tabular numerical representation of mortality and survivorship of a cohort at each age of life. Most life tables are based on current mortality rates, and such tables are called period, current or cross-sectional life tables.
Period life tables assume that as a cohort passes through life, it experiences the age-specific mortality rates of a given period. Although these tables are based on death rates of the population during a particular period of time, they are a hypothetical model of mortality as they do not describe the real mortality which characterises a cohort as it ages.
In contrast, cohort (or generation) life tables are based on the actual mortality experience of a particular cohort (eg all people born in the year 1900). These tables require data over many years, theoretically until the death of the last survivor.
Every five years, Statistics New Zealand produces complete period life tables using average mortality rates for three successive years centred on a census year. A complete life table presents functions for each single-year of age.
In every non-census year, Statistics NZ produces abridged period life tables using mortality rates for three successive years centred on a non-census year. These abridged life tables are an interim indication of mortality and survival trends of the total New Zealand population until complete period life tables are derived. An abridged life table presents functions for five-year age groups rather than by single-year of age, although age 0 and 1–4 years are identified separately.
In 2006, Statistics NZ released complete cohort life tables for the New Zealand population for the first time. These track the mortality and survival trends of people born in each year from 1876, and are updated and extended annually.
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 may identify with the 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 does not equate to a birthplace description.
Changes to birth and death registration forms
In September 1995, new birth and death registration forms were introduced carrying a revised question on ethnicity. Previously the ethnic question on the death registration form asked for the degree of Māori or Pacific Island blood, if any, of the deceased person's parents. The new ethnic question, which is the same in principle to the ethnic question used in the 1996 Census, instructs the respondent to "tick as many circles as needed to show which ethnic group(s) deceased belonged to". This has resulted in a number of changes:
- the ethnic concept is now self-identified ethnicity (previously Māori and Pacific respondents were classified by their 'degree of blood')
- ethnic vital statistics are now available for a wider range of ethnic groups (previously information was only sought for Māori and Pacific groups)
- ethnic data is now directly available for newborn babies and the deceased (in both cases this was previously derived from their parent's ethnicity)
- multiple response to the ethnicity question is now possible
- non-response to the ethnicity question can now be quantified.
In the case of births and deaths, ethnicity is identified by the person completing the registration form. For births this is usually the parent(s), while for deaths this is most likely to be the funeral director (on the advice of a family member).
Availability and comparability of ethnic life tables
Statistics NZ has produced period life tables for the Māori and non-Māori populations in addition to period life tables for the total New Zealand population. Following the introduction of new birth and death registration forms in September 1995, Statistics NZ has also evaluated the production of life tables for other ethnic groups such as Asian and Pacific. However, official life tables for other ethnic groups have not been produced because of the relatively small size of these ethnic populations, relatively few death registrations, and uncertainty associated with ethnic identification and measurement. For example, a person's ethnic identity can change over time and between different data collections, depending on the respondent and the context of collection. Hence, Statistics NZ is not yet confident that life tables provide a statistically robust measure of the mortality and survival experience of these ethnic populations for a given period (eg 2005–07) or over time (eg between 2000–02 and 2005–07), other than for Māori and non-Māori. All ethnic mortality measures, including those for Māori and non-Māori, should be interpreted with due caution.
Customised and non-official life tables for the Pacific and 'European or Other (including New Zealander)' ethnic groups are available on request from Statistics NZ. For more information and quotes email demography@stats.govt.nz.
There is evidence that Māori deaths were significantly under-reported, and hence non-Māori deaths were over-reported, before the new vitals registration forms were introduced in 1995. Following the redesign of the forms, death registrations and population data are broadly comparable for ethnic groups. The 1995–97 Māori and non-Māori life tables published in July 1998 were the first to be constructed using data derived from the new registration forms. However, because numerator-denominator ethnic differences are significant before 1995–97, ethnic mortality measures from 1995–97 are not comparable with those from earlier years. For alternative estimates of life expectancy of different ethnic populations before 1995–97, see Ajwani (2003). Note, however, that these estimates may not be directly comparable to mortality measures produced by Statistics NZ because of differences in both the deaths numerator and population denominator.
Method
The data used to construct the 2005–07 life tables comprises:
- deaths registered in New Zealand of people resident in New Zealand in the December years 2005–07, respectively, by single-year of age, sex and ethnicity
- the estimated resident population of New Zealand at 30 June 2006 by single-year of age, sex 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. Hence, 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 has been made. There was no response to the ethnicity question for 4.9 percent of deaths in 2005–07.
Because deaths in the first year of life are skewed towards the early part of this age, infant death rates are calculated from more detailed data. This involves the division of the first year of life into more detailed ages. The reports New Zealand Life Tables, 2000–2002 (Statistics NZ, 2004) and New Zealand Period Life Tables: 2005–07 (Statistics NZ, forthcoming) describe the methodology in more detail.
Population denominator
The estimated resident population at 30 June 2006 is based on the census usually resident population count at 7 March 2006 with adjustments 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.
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 ethnic group 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 and 2001 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 and 2000–02 periods. Note that all population estimates used in the 1995–97 and 2000–02 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 on the Statistics NZ website (www.stats.govt.nz).
Usually the mean population of each 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 population at 30 June (the midpoint) of each period is used. The impact of using 'midpoint' rather than 'mean' population estimates is generally insignificant.
Derived rates
The life tables are 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 is a small number of deaths and at very old ages where the population at risk is small. In some cases the New Zealand data does not enable death rates to be reliably estimated at all ages. 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, the United Kingdom and the United States. For the Māori and non-Māori populations, death rates have also been modelled at some younger ages.
There are some small observed numerator-denominator ethnic differences since 1995 in comparison with 1996, 2001 and 2006 census data. For the 2000–02 and 2005–07 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). For the non-Māori 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.
For more information about life tables, refer to Information about the life tables on the Statistics NZ website (www.stats.govt.nz).
Forthcoming information
Statistics New Zealand produces abridged period life tables on an annual basis. These are not as detailed as complete life tables, as abridged life tables present functions for grouped ages rather than single years of age, and are not derived by ethnicity. Abridged period life tables for 2006–08 will be available in February 2009 (provisional) and May 2009 (final).
More details on life table methodology and results relating to New Zealand subpopulations will be included in the publication New Zealand Period Life Tables: 2005–07 which will be released in the first half of 2009.
Definitions
Cohort: a group of people sharing a common demographic experience. For example, the 1900 birth cohort refers to the people who were born in the year 1900.
Estimated resident population: an estimate of all people who usually live in a given area at a given date. The estimated resident population of New Zealand 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), and an adjustment for residents missed or counted more than once by the census (net census undercount). Visitors from overseas are excluded.
The estimated resident population at a given date after census includes births, deaths and net migration (arrivals less departures) of residents during the period between census night and the given date.
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.
Life expectancy at birth: the average length of life of a birth cohort. As derived from a period life table, it is the average length of life of a newborn baby, assuming they experience the age-specific mortality rates of a given period throughout their life.
Life table: a tabular numerical representation of mortality and survivorship of a cohort of births at each age of life.
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.
The period life tables presented here assume that as a cohort passes through life it experiences the age-specific mortality rates of a given period. Although they are 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 which characterises a cohort as it ages.
Māori population: as used here, refers to people who identify with the Māori ethnicity, with or without other ethnicities. Because ethnicity is self-perceived, people can identify with Māori ethnicity even though they may not be descended from Māori ancestors. Conversely, people may choose to not identify with Māori ethnicity even though they are descended from Māori ancestors.
Mean population: the average number of people in an area during a given period, usually a year. This measure may be estimated in terms of a simple or weighted arithmetic mean of monthly or quarterly population during the reference period.
References
Ajwani S et al (2003). "Decades of Disparity: Ethnic mortality trends in New Zealand, 1980–1999", Wellington: Ministry of Health and University of Otago, Public Health Intelligence Occasional Bulletin Number 16. www.moh.govt.nz/moh.nsf/pagesmh/2355
New Zealand Health Information Service (2007). "Mortality and Demographic Data 2004", Wellington: Ministry of Health. www.nzhis.govt.nz/moh.nsf/pagesns/528
OECD (2008). OECD Health Data 2008. stats.oecd.org/WBOS/Index.aspx
Statistics New Zealand (1996). "Recent changes to ethnic statistics on births and deaths", Demographic Trends 1996, p145–147.
Statistics New Zealand (1997). "New ethnic-specific birth and death rates", Demographic Trends 1997, p153–155 part 10. www.stats.govt.nz/analytical-reports/dem-trends-97
Statistics New Zealand (2004). "New Zealand Life Tables, 2000–2002", Wellington. www.stats.govt.nz/analytical-reports/nz-life-tables-2000-2002
Statistics New Zealand (2006). "A History of Survival in New Zealand: Cohort life tables 1876–2004", Wellington. www.stats.govt.nz/analytical-reports/history-survival-new-zealand