Population Pyramid

A population pyramid 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 normally forms the shape of a pyramid. It typically consists of two back-to-back bar graphs, with 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.
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 its development.
Triangular population distribution The triangular distribution may also be called a pyramid or exponential(expansive) distribution. The wide base indicates a large number of children but the rapid narrowing shows that many people die between each age band. The pyramid indicates a population in which there is a high birth rate, a high death rate and a short life expectancy. This is the typical pattern for less economically developed countries due to little access to and incentive to use birth control, negative environmental factors (for example, lack of clean water) and poor access to health services.
Note that there tend to be more females than males in the older age groups, due to females' longer life expectancy.

[edit] Types of population pyramid While all countries population pyramids differ, three types have been identified by the fertility and mortality rates of a country.
Stationary pyramid - A population pyramid showing an unchanging pattern of fertility and mortality.
Expansive pyramid - A population pyramid showing a broad base, indicating a high proportion of children, a rapid rate of population growth, and a low proportion of older people.
Constrictive pyramid - A population pyramid showing lower numbers or percentages of younger people.
Uses of population pyramids Population pyramids can be used to find the number of economic dependents being supported in a particular population. Economic dependents are defined as those under 15 (children who are in full time education and therefore unable to work) and those over 65 (those who have the option of being retired). Of course, in some less economically developed countries children start work well before the age of 15, and in some more economically developed countries it is not usual to start work until 18 or 21, and people may work beyond the official retirement age of 65, but the definition provides an approximation. The government must plan the economy in such a way that the working population can support these dependents.