Guide To Mortality Measures

Early neonatal, neonatal, post-neonatal, and perinatal death rate

These rates measure the number of deaths at different ages of infancy during a year among 1,000 babies born in that year.

• Early Neonatal Mortality Rate: Number of early neonatal deaths (deaths under 7 days of age) per 1,000 live births.
• Neonatal Mortality Rate: Number of neonatal deaths (deaths under 4 weeks of age) per 1,000 live births.
• Post-neonatal Mortality Rate: Number of post-neonatal deaths (deaths between 4 weeks and 1 year of age) per 1,000 live births.
• Perinatal Mortality Rate: Number of stillbirths plus infant deaths under 7 days (early neonatal deaths) per 1,000 total births.

As with the IMR, limitations arise because of inconsistencies between the deaths numerator and population denominator. For example, some of the deaths during a year are of infants born during the preceding year, and some of the infants born during the year will die during the following year before they have reached one year of life. Furthermore, when these rates are based on death and birth registrations during a period, lags in registration can cause misleading patterns.

Fetal death rate

The fetal death rate is the number of fetal deaths in a year per 1,000 total births (stillbirths plus live births). Total births are generally based on the registrations of stillbirths and live births in a year, rather than on the number of births occurring in the year.

Fetal death statistics are typically based on registrations of stillbirths (late fetal loss) only, defined as a child who showed no signs of life at the time of birth and either:

• weighed 400g or more at birth, or
• was born after the 20th week of pregnancy.

Before September 1995, a stillbirth was defined as "a child who was not alive at the time of birth and was issued from its mother after the 28th week of pregnancy".

Fetal and infant deaths web tool provides fetal death rates based on stillbirths (Health New Zealand / Te Whatu Ora publication). Not all stillbirths are registered, with some information collated from stillbirth cause of death data, resulting in a slightly longer lag in timeliness than the Stats NZ data.

Standardised death rate

Standardisation is a technique to remove or control for a factor, such as age, to allow meaningful comparisons of different population groups which may have very different compositions (such as age structures).

The standardised death rate (SDR) is the overall death rate that would have prevailed if a standard population experienced the age-and-sex-specific death rates of the population being studied. Stats NZ publishes an annual SDR for the total and Māori ethnic population that uses the direct method of standardisation and the age and sex distribution of the mean estimated population for the year ended 31 December 1961 as the standard.

The SDR is less useful as a standalone measure, but is most useful when comparing different time periods or different populations, provided the SDRs have used the same standard population.

For example, the crude death rate (CDR) of the total population was 7.3 per 1,000 in 2022−2024, which was higher than the CDR of the Māori ethnic population of 5.3 per 1,000. However, the respective SDRs were 3.5 and 6.0, indicating that once accounting for different age-sex structures, Māori death rates were higher. This is also confirmed by life tables that indicate the period life expectancy of the total population (males and females combined) was 81.8 years, higher than that of the Māori ethnic population at 75.8 years.

Life expectancy

Stats NZ produces two measures of life expectancy (the average length of life remaining at a given age) - period and cohort. The calculation of life expectancy relies on the derivation of a life table.

Life tables are derived by applying death numbers to the population 'at risk' at each age to give death rates, from which various probabilities of dying and surviving, and ultimately life expectancies, can be calculated. The derivation of life tables is beyond the scope of this guide, but see New Zealand period life tables: Methodology for 2017-2019 (also applicable to the 2022─2024 period life tables). That paper also provides an overview of life table functions.

Period life expectancy

Period life expectancy is the average length of life remaining at a given age, assuming people experience the age-specific death rates of a specific period from the given age onwards. For example, life expectancy at birth for the period 2022-2024 is based on death rates for that period, and takes no account of changes in death rates after 2022-2024.

Period life expectancy is therefore a hypothetical construct. Although period life expectancy is a timely and useful summary of relative mortality levels over time or between populations (for example, males and females, Māori and non-Māori, different areas of New Zealand, different countries), it will typically under-estimate expected lifespans (if death rates continue to decline).

Period life tables can be separated into:

• complete - where single-years of age are presented
• abridged - where 5-year (or 10-year) age groups are presented, although age 0 and 1─4 years are typically separated.

Stats NZ typically derives complete period life tables every 5 years, for national-level total and major ethnic populations. Mortality and life expectancy tend to change gradually over time, and this frequency enables census-based population statistics to be used as the 'at risk' population denominator. The life tables cover a three-year period to minimise the impact of year-to-year fluctuations in deaths.

Stats NZ derives abridged period life tables each year for the national-level total population. These provide an interim indication of trends after complete period life tables are derived, but the abridged life tables are not revised to account for revised population estimates.

Annual period life expectancy is therefore not explicitly published, but it is derivable by transposing published cohort life table data (figure 5).

Figure 5

Life tables are typically derived for males and females separately, because their mortality rates are different. For someone wanting life expectancy for males and females combined, an average of the male and female life expectancy is sufficient for most purposes. A more refined life table, which accounts for the different numbers of males and females at each age, would not be majorly different from the average, for a population with roughly similar numbers of males and females across the ages.

For sub-populations, there is often a question of which life tables provide the best comparison. Comparisons with the total population life tables are common. It can also be appropriate to compare with a non-overlapping population to avoid masking the true mortality differentials. For example, comparisons between Māori life tables and non-Māori life tables provide a comparison between those who identify with Māori ethnicity and those who do not, without the conflation of Māori becoming an increasing share of the total population (deaths and population) over time.

Cohort life expectancy

Cohort life expectancy is 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. For people born on 1 January 1900, it would be based on death rates for age 0 in 1900, for age 1 in 1901, for age 2 in 1902, and so on.

Cohort life expectancy measures the mortality experience of a particular cohort (for example, all people born in 1900), by following that cohort from birth to death (that is, until the death of the last survivor). Since cohort life expectancy is based on the death rates of the same group of individuals over their lifetime (allowing for people leaving and joining the cohort through migration), it presents a true historical record of mortality experienced by a birth cohort.

The New Zealand cohort life tables allow for people leaving and joining the cohort through migration, so relate to an 'open' cohort not a 'closed' cohort.

The questions 'What lifespan did a population experience?' and 'What lifespan should I expect?' are therefore best answered by using the cohort life expectancy of a specific cohort. For birth cohorts that are still alive, some projection of remaining mortality experience is needed to enable calculation of cohort life expectancy.

A history of survival in New Zealand: Cohort life tables 1976-2004 details the compilation of the cohort life tables, which are updated and extended annually.

Example of 1961 birth cohort

The 1960-1962 period life tables indicate the period life expectancy (at birth) of a newborn girl as 73.8 years (figure 6). This was derived from the death rates of the specific period 1960-1962, which are based solely on those who died during that period.

The 1961 cohort life table indicates the cohort life expectancy (at birth) of a girl born in 1961 as 83.0 years - 9 years more than the period life table figure. In fact, most (about 90 percent) of the 1961 birth cohort are still alive as of early 2026, so death rates at remaining ages (above mid-60 years) need to be projected to calculate the cohort life expectancy. This makes cohort life expectancy more difficult to calculate, as it needs an extensive historical series of death rates and/or plausible projections of future death rates at each age.

Figure 6

How long are New Zealanders living?

The cohort life tables answer some fundamental questions about the longevity of New Zealanders.

For males and females born in the late 1870s, life expectancy at birth was about 51 and 55 years, respectively (figure 6). Life expectancy has progressively increased since - except for males born around the early 1890s and late 1910s who were most affected by deaths during the two world wars. (The period life tables do not include war deaths, which largely occurred overseas.)

For males and females born in the mid-2020s, life expectancy is projected to be around 84 to 92 and 88 to 95 years, respectively - there is significant uncertainty because these cohorts are just beginning their lives.

How long will I live? combines historical data from 1876 with the latest mortality assumptions from national population projections to give an indication of the age you're likely to live to (cohort life expectancy). These life expectancies apply to each birth cohort, not to individuals.

New Zealanders reaching age 65 can now expect to live into their late 80s

For those reaching age 65 in 2025 (born in 1960), the cohort life tables indicate a life expectancy around 85.9 years for males and 88.3 years for females (figure 7). That is, a further 20 to 22, and 23 to 24 years, on average, for males and females, respectively. These tend to be longer than most people expect, because their expectations of lifespan are often based on the age at death of their parents, or of other family and friends, or of those currently dying.

Figure 7