Population Pyramid

The population pyramid is a graphical representation of the age and gender composition of a specific population. The shape of the graph depends on the age and gender structure of the population. The representation may take the form of a pyramid, but it may have a columnar shape, with vertical sides rather than sloped sides, or it may have an irregular profile.

Population pyramids provide a summary view of the overall age-gender structure of a specific population. The size of the population is depicted on the horizontal axis, and age is aligned on the vertical axis. The depiction actually contains two graphs, in mirror image format, on either side of a central vertical axis; the female population is represented on the right side of the axis, and the male population is shown on the left side.

The population pyramid is made up of bars stacked on top of one another, each representing an age category, typically in 5-year age groups, with the youngest age group represented by the bottom bar, and the oldest age group by the uppermost bar. The length of each bar, on either side of the central vertical axis, [Page 823]represents the number of males (left side) and females (right side) in the specific age group, in the population depicted. The age groups are displayed along the central axis or along one side, and often the years of birth for each age category are also displayed on the graph. To maintain proportionality, the age groups are all of the same size (typically in 1-year, 5-year, or 10-year age groups), and the bars are all of equal height. However, the age axis is often truncated at the age group 80 to 84, depending on the data available for the population depicted. For some populations, the data for the older age groups are incomplete or inaccurate, or there are few people in the older age categories.

Figure 1 Age-Sex Structure of Global Population: 2002

Source: U.S. Census Bureau (2004a).

The population pyramid can depict the proportion of the total population in each age-gender group rather than the actual count. In this case, for example, the length of the bar for females in the 5to 9-yearold group would represent the proportion that group consists of within the total population. When calculating the proportions, the denominator used is always the number in the entire population, and the numerator is the number in the specific age-gender group. The sum of all the groups represented by the bars should add up to 100% of the population depicted.

When comparing population pyramids, it is important to note whether proportions or counts are represented and whether the scale of the bars and the age categories are the same. Population pyramids intended for comparison should be drawn to the same scale, and should depict the same age categories. The population pyramid can be used to represent additional characteristics of the population, such as marital status, race, or geographic location. In this case, the bar for each agegender group is further subdivided and formatted to represent the additional categories. The formatting system used to depict the additional categories should be applied consistently throughout the graph. The same sequence should be used on either side of the vertical axis, in mirror image form. For example, if race is depicted, and the categories are white, black, and other, the categories would be arranged in the same sequence for males and for females, working outward from each side of the central axis.

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