Visualising the demographic factors which shape population age structure

Abstract

1. IntroductionThe population pyramid is one of the most popular visual representations of data in demography. In its standard form it comprises two histograms rolled on their sides and placed back-to-back, with the youngest ages at the bottom of the diagram and the oldest at the top. Generally, the male population is placed on the left and the female population on the right. The populations of each age-sex group are shown either as absolute numbers or as a percentage of the whole population. As an example, Figure 1 presents a population pyramid for Australia at 31 December 2014.A population pyramid can be created in common spreadsheet or statistical software without much difficulty, and is easy to understand. It presents immediately digestible information on the age-sex structure of a population that would be less obvious in tabular form. This distribution is important for understanding the demand for the wide range of goods and services which vary by age and sex, such as baby products, childcare, education, housing for first home buyers, household appliances, recreational activities, aged care, and funeral services (Siegel 2002). Population age structure also affects government spending and taxation receipts (Australian Government 2015). A population pyramid additionally offers clues about a population's fertility history, mortality and migration, position in the demographic transition, and the likely influence of its age-sex structure on future demographic change. The shape of a population pyramid can hint at the economic or demographic role of the population in question, as might be apparent for a university town, a popular seaside retirement region, or an area with a communal establishment like a boarding school or prison. It can also be used to check for potential data problems, such as age heaping. Finally, population pyramids can also be quite entertaining, especially in dynamic form, as presented on some national statistical offices' and researchers' websites. Examples can be found* for Germany,* for Australia, and* for Moldova.Several variations on the standard pyramid have been created. Many involve disaggregating the bars into different categories of population, such as by marital status (e.g., Statistics Canada 2013), household living arrangement (e.g., van Imhoff and Keilman 1991), birthplace (e.g., Statistics Sweden 2009), educational attainment (e.g., Lutz, Cuaresma, and Sanderson 2008), labour force status (Statistics New Zealand 2015), ethnicity (e.g., Coleman 2010), and migrant status (e.g., Price 1998). Some display shadows on the bars of the pyramid where there is an excess number of one sex over another (e.g., Heenan 1965); others show age-sex structure with persons by age in one histogram and age-specific sex ratios in another (Haak 1942). Commonly, two or more population pyramids are overlaid on one another, often to compare age structure over time (e.g., The Economist 2011), to show the outcome of alternative future scenarios (e.g., Statistics Canada 2015), or to display probabilistic prediction intervals (e.g., Keilman, Pham, and Hetland 2002).However, population pyramids are not without their limitations. Because they display only the size of each age-sex group in the population, it is usually not possible to distinguish the relative contributions of different demographic processes to population age structure. For example, if there is an indentation at the young adult ages, is this more the result of a fertility decline two to three decades earlier or recent net out-migration? If age-specific populations at the upper end of the pyramid decline very rapidly with increasing age, is this just the result of mortality, or have past trends in births and migration also contributed?This paper presents a type of population pyramid that illustrates how births, deaths, and net migration have shaped a population's age structure. Termed the components-of-change pyramid, it is described and illustrated in section 2 following this introduction. …