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Steps to follow when analysing population structure
Before we can analyse the implications of a population’s structure, we need to understand the population structure and the factors that will cause it to change. This section provides a framework for analysing structure, describes the causes of change to population structure, and illustrates the age-sex pyramid that is a useful tool for describing population structure and change.
Framework
1. Define the population with which we are concerned (eg number of people with the relevant characteristics in a set area).
2. Select relevant population information. This may include:
- age and sex structure of the population
- information on families and households
- social and economic context
- expected trends in projected population change.
3. Describe how the elements of the structure interrelate (eg age-sex profile, ethnicity).
4. Analyse the problem in the context of the structure.
Framework components
1. Population structure
Population structure can be described in terms of:
- the number of people involved
- their characteristics (especially their age, sex and ethnicity)
the processes which change the make-up of the population over time.
For some limited purposes, a description of these basic demographic features will suffice. But a more sophisticated understanding of how populations change over time requires close attention to the specific characteristics (like age and sex) of the different groups that make up the population. Once these characteristics are described, how they will be affected by change can be examined.
2. Causes of change to population structure
Three components of growth change population structure over time:
People are lost to the population either by death or by out-migration. People are added to the population either as births or in-migrants.
Births
Births add only to the youngest cohort of a population. The number of births is affected by the number of women in the childbearing age groups as well as the timing of childbearing and the number of children women and their partners choose to have.
Deaths
Deaths tend to affect the youngest and the older cohorts more than other cohorts. In New Zealand, infant mortality has dropped to low levels and life expectancy for people in the older ages has increased. This affects both the number of babies surviving into childhood and the number of people in the oldest age groups. Changes in healthy life expectancy affect the level and nature of demand for care services among older age groups.
Migration
Migration affects all ages. Because the age and sex profile of out-migration from an area or from the country differs from the profile of in-migration, cohorts are affected in different ways. While net migration (the difference between the in- and out-migration of a population) affects population size, the difference between the inflow and outflow profiles affect the population structure.
In addition, migration patterns change over time, which means that the nature and scale of these effects also change over time.
3. Describing population structure
Age-sex pyramid
An age-sex pyramid is a graphical representation of a population’s age and sex structure. It is a snapshot of a population’s structure at a particular time, that can be measured against historical snapshots of the population’s structure to illustrate the process of change in the composition of the population over time.
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