Each year, PRB releases its World Population Data Sheet, a globally recognized report offering population projections and fertility rates for almost all countries around the world. A public information product accessible to anyone, the Data Sheet reaches tens of thousands of people worldwide who count on it for essential data and insights. We will release this year’s World Population Data Sheet in August 2018.

In addition to providing readers and researchers with global population and fertility data, each edition of the Data Sheet highlights a specific topic. Past issues have focused on wealth and income inequality; demographic, health, and environmental progress and challenges; women’s empowerment; human needs and sustainable resources; and youth. In honor of World Population Day, we offer a sneak peak of the 2018 World Population Data Sheet focus: Changing Age Structures.

To understand changing age structures, you need to know three essential terms: age structure, dependency ratio, and demographic transition.

What Is Age Structure?

Age structure is the proportion of the total population in each age group. Population aging—the process in which the proportions of working age and older adults increase in a population, while the proportions of children and adolescents decrease—is measured by the share of older people in the population.

What Is a Dependency Ratio?

A dependency ratio is the ratio of people in a dependent age group (those under age 15 or those age 65 and older) to those in the economically productive age group (ages 15 to 64) of a population. A child dependency ratio of 0.45, for example, means there are 45 children for every 100 working-age adults.

What Is Demographic Transition?

The demographic transition is the historical shift in birth and death rates from high to low levels in a population. Declining mortality usually precedes declining fertility, resulting in rapid population growth during the transition period.

With these definitions in hand, our 2018 World Population Data Sheet will be easy to understand when we launch it this August. Welcome to the fascinating field of population studies!