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A population pyramid, also called an age pyramid or age picture diagram, is a graphical illustration that shows the distribution of various age groups in a population (typically that of a country or region of the world), which forms the shape of a pyramid when the population is growing.〔( Population pyramids of the world from 1950 to 2050 )〕 It is also used in ecology to determine the overall age distribution of a population; an indication of the reproductive capabilities and likelihood of the continuation of a species. It typically consists of two back-to-back bar graphs, with the population plotted on the X-axis and age on the Y-axis, one showing the number of males and one showing females in a particular population in five-year age groups (also called cohorts). Males are conventionally shown on the left and females on the right, and they may be measured by raw number or as a percentage of the total population. Population pyramids are often viewed as the most effective way to graphically depict the age and sex distribution of a population, partly because of the very clear image these pyramids present.〔(Department of Health Home )〕 A great deal of information about the population broken down by age and sex can be read from a population pyramid, and this can shed light on the extent of development and other aspects of the population. A population pyramid also tells how many people of each age range live in the area. There tends to be more females than males in the older age groups, due to females' longer life expectancy. ==Types of population pyramid== While all countries' population pyramids differ, four general types have been identified by the fertility and mortality rates of a country.〔http://www.fsl.orst.edu/pnwerc/wrb/Atlas_web_compressed/5.Human_Populations/5h.pyramids_web.pdf Population Pyramids - Oregon State University〕 ;Stable pyramid :A population pyramid showing an unchanging pattern of fertility and mortality. ;Stationary pyramid :A population pyramid typical of countries with low fertility and low mortality, very similar to a constrictive pyramid. ;Expansive pyramid :A population pyramid that is very wide at the base, indicating high birth and death rates. ;Constrictive pyramid :A population pyramid that comes in at the bottom. The population is generally older on average, as the country has long life expectancy, a low death rate, but also a low birth rate. This pyramid is becoming more common, especially when immigrants are factored out, and is a typical pattern for a very developed country, a high level of education, easy access to and incentive to use birth control, good health care, and few negative environmental factors. In some countries the above "pyramids" are referred to by a description of their shape. The classic one is of course the pyramid. The stationary pyramid is referred to as a "clock-model" (like the bell in a clock tower). The contracting pyramid is referred to as "onion" shaped. Sometimes even as "urn" shaped. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「population pyramid」の詳細全文を読む スポンサード リンク
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