Dependency ratio is determined by the age structure of a population. It is a measure of the number of dependents (people who are not in the labor force) in relation to the number of working-age people.
The dependency ratio is calculated by dividing the number of dependents (typically children under 15 and adults over 65) by the number of working-age people (usually defined as those aged 15-64). The result is expressed as a ratio or percentage.
For example, a country with a high proportion of young and old people relative to the number of working-age adults will have a high dependency ratio. This means that there are relatively more people who are dependent on those who are working to support them. Conversely, a country with a lower proportion of dependents relative to the number of working-age adults will have a lower dependency ratio, meaning that there are relatively fewer people who are dependent on those who are working to support them.
Factors such as the rate of natural increase (births minus deaths) and net migration can impact the age structure of a population, which in turn can affect the dependency ratio. However, the age structure of the population is the primary determinant of the dependency ratio.