Age-specific rates | Age-sex pyramid | Age composition | Ageing | Birth cohort | Census of population and dwellings | Cohort | Cohort analysis | Cohort component method | Crude rates | Demographic characteristics | Demographic transition | Dependency ratios | Ethnicity
Age-specific rates
A measure specific to an age group. Rates for each age or age group are commonly calculated for fertility, mortality, marriage, marriage dissolution, and abortion because rates vary greatly through a person's lifetime.
Age-specific rates may be expressed in terms of a five-year age group (0-4, 5-9, 10-14, 15-19, 20-24, 25-29 years, etc), single-year of age, or other categories relevant to the particular measure, such as school age, working age etc.
Age specific rates should always be considered alongside average measures for a population, especially if two populations with different age structures are being compared (see also standardisation for age for a technique for adjusting for age structure differences).
Age-sex pyramid (also referred to as a Population pyramid)
A bar chart graphically representing the age structure of males and females for a population. Commonly age-sex pyramids are constructed using five-year age groups, though these may not be as informative as single-year-of-age pyramids – as with all graphical representation the aim should be to inform the user, so the exact format needs to reflect this aim.
The Population and Sustainable Development Report (pp 13-15) contains examples of population pyramids for New Zealand’s total population, and for different ethnic groups.
The age structure of the population usually approximates the shape of a pyramid because mortality progressively reduces the number in each birth cohort as it ages. The age pyramid is useful to show the overall shape of the population, the relative sizes of individual cohorts within the population. Some inferences can be drawn from this about the past and the future of the population.
Age composition
Refers to the proportion of the population who are in each age group or key life cycle stage. The age structure of the population largely reflects past patterns of fertility and migration. In New Zealand, implied consequences of low fertility rates and very low, or even negative, natural increase mean that changes in age structure are becoming increasingly important for policy and planning.
Ageing
Population ageing occurs when there are fewer babies born and a smaller net gain of people younger than median age through migration than there are losses due to people older than the median age dying or being lost through international migration. This results in an increased proportion of the population in older age groups and a higher median age.
Sometimes ageing is referred to as structural ageing in contrast to numerical ageing, but this latter term refers to population growth without change in age structure, and therefore is not referring to population ageing but to a larger number of people in the older ages.
Birth cohort
The people alive who were born within the same year or other specified period. The size of a birth cohort is related to the number of women in the main reproductive age groups who give birth in that year but is modified over time by international migration and mortality.
Examples of analysis based on birth cohorts are the Dunedin Multidisciplinary Health and Development Study that started in 1973, the Christchurch Health and Development Study that commenced in 1977 and the recently initiated Pacific Families Study at Auckland University of Technolgy.
Census of population and dwellings
The five-yearly Census of Population and Dwellings is the official count of population and dwellings in New Zealand, providing a snapshot of the population and society at a point in time. The census provides information on people who were in New Zealand on census night and were enumerated - their age, sex, education, ethnic groups, employment, where they live, and so on. This includes information on dwellings and on visitors from overseas.
Comparative analysis of censuses over time can, with care, provide very detailed information on trends. However, for information on population size and location, it may be more appropriate to use the official population estimates and population projections produced by Statistics New Zealand. To avoid confusion with other censuses (such as the agricultural census), it is advisable to refer to the Census of Population and Dwellings by the full title where it is not clear.
Cohort
A group of people sharing a common demographic experience. The most common cohort is a group of people born in the same year (birth cohort), but there are numerous other examples, such as those who, in the same period of time, married (marriage cohorts), went on a benefit (benefit entry cohort) or migrated (migration cohorts).
Cohort analysis
The tracing of the experiences of groups of people over successive years. The cohort is a core concept in demography that has many applications in public policy and planning. A cohort is a group of people connected by a similar event.
Cohort analysis tracks the changing numbers and characteristics of a cohort over successive years. Comparison of different cohorts can reveal the impact of the particular circumstances in which they lived. Some differences between cohorts reflect gradual changes over time – such as changes in women's employment – while others reflect the concentrated effects of some events, such as wars, on particular cohorts.
Cohort analysis is contrasted with period analysis (or cross-section analysis). Period analysis entails working with data for different age groups at the same point in time (the same date or year). Both cohort and period analysis are useful, though one may be more appropriate than the other in particular situations.
Cohort component method
This method of estimating or projecting the population advances each cohort within the base (or initial/starting) population by age, accounting for expected births and deaths and adjusting for international migration. In deriving population estimates, actual events on births, deaths and international migration are used where these are available.
For population projections, the base population is projected forward by calculating the effect of deaths and migration within each age-sex group according to specified mortality and international migration assumptions. New birth cohorts are generated by applying specified fertility assumptions to the female population of childbearing age. The cohort component method is also known as the cohort survival method.
Crude rates
The number of vital events which occurred in a population in one year are divided by the size of the population at the middle of that year (or mean population over that year). The rates are said to be crude because no account of composition effects is made, unlike rates based on age-specific data.
Demographic characteristics
The characteristics of a human population such as sex, age, marital status, ethnic origin, education, income, religion, and place of residence.
Demographic transition
The shift from high fertility and high mortality rates to low mortality and subsequently low fertility rates. During the transition itself, rapid population growth often occurs (as in many developing countries today) because the death rate falls before birth rates fall.
The main outcome of the demographic transition is a shift from a population that is youthful and to one that has an older profile. In the course of the transition, a population may temporarily exceed its ultimate size so that it undergoes a period of population decline before stabilising to its eventual shape.
The current situation in many OECD countries, including New Zealand, of below replacement fertility is viewed by some as representing a second demographic transition, though the long-term consequences of this are yet to be clarified.
Dependency ratios
Dependency ratios provide simple summary measures of age composition, with respect to relative numbers of people in “dependent” and “productive” groups.
The ratios are typically based on a division of the age range into three broad, somewhat crude groupings: children (0-14), working ages (15-64), and older people (65+ years). Varying these ranges can affect the ratios significantly.
Moreover, variations in the size of cohorts can lead to rapid changes in the short-term and a general shift from child-dependency as the dominant component of total dependency to aged-dependency as the dominant component in the longer-term.
Common dependency ratios used to measure support needs of a population include:
- the child dependency ratio (the number of dependent children per hundred people of working age)
- the aged dependency ratio (the number of older people per one hundred people of working age)
- the economic dependency ratio (the number of people not in the labour force per one hundred in the labour force)
- the ageing index (the number of aged people per one hundred children), and
- the caregiver ratio (the number of people aged over 80 years per one hundred females aged 50-64).
Because definitions change, it is important to carefully specify the composition of the categories (for example, working ages may have a range of end points and these change over time).
Ethnicity
The ethnic group or groups that people identify with or feel they belong to. An ethnic group is a social group whose members have some or all of the following characteristics:
- share a sense of common origins
- claim a common and distinctive history and destiny
- possess one or more dimensions of collective cultural individuality
- feel a sense of unique collective solidarity.
Ethnicity is self-perceived and people can belong to more than one ethnic group. Ethnicity is synonymous with neither ancestry nor race: people can identify with an ethnicity even though they may not be descended from ancestors with that ethnicity. Conversely, people may choose not to identify with an ethnicity even though they are descended from ancestors with that ethnicity. Similarly, people may change their ethnicities over time or in different social contexts. For information on issues related to ethnicity see here.
While all data collections should collect ethnicity directly from an individual, it is not always possible (for example for births and deaths). Care is needed when comparing data collected by self-identification with data collected from a proxy.