Population Pyramids

A population pyramid shows the distribution of a variable across categories. It is two back-to-back histograms (when the distribution variable is scale) or two back-to-back bar charts (when the distribution variable is categorical), with the bars in the chart being horizontal rather than vertical. When there are more than two categories, creating the chart results in more than one population pyramid, depending on the number of categories. For example, if there were four categories, there would be two population pyramids, one for each category pair.

Population pyramids are often used for demographic data. A common population pyramid displays the counts for age groups in each gender, with the youngest age at the bottom. A viewer can easily distinguish differences among the age groups and between genders because of the different bar lengths and the symmetry between halves.