Population Sector


Purpose and Perspective

Because population levels and growth rates have a major influence on development, a long-term model of the development process requires endogenous treatment of both fertility and mortality. Also the model should capture the dynamics of major factors that influence fertility and mortality, and the principal ways in which population and its growth rates influence development.

The population sector simulates total population and population age distribution based on endogenous fertility and mortality.

In the iSDG Population sector, the population stock is an accumulation of three flows: births, deaths and net migration. The Population sector is a coherent component model with 101 age cohorts (age 0 to 100+) for both genders.

While population data from many sources comes in five-year age cohorts (in particular UNPOP) [2], we use a one-year age cohort structure because it gives us greater flexibility and accuracy. This flexibility is especially important for handling age-specific matters in other sectors of the model. For example, countries and organizations use different definitions of the ages covered by “infant mortality rate.” Also, the age at which children enter school varies by country.

The flexibility of using one-year age cohorts in iSDG makes for easy application of the model in any country and does not significantly complicate the programming or data input.

Sector Structure and Major Assumptions

  • Migrants have the same fertility and mortality behavior as the rest of the population [3]

Exogenous Input Variables

  • Net Migration Per Thousand People - Units: Dmnl [4]/Year

Initialization Variables

  • Initial Population[Female, Age] - Units: Person

  • Initial Population[Male, Age] - Units: Person

  • Initial Population Birthday Distribution - Units: Dmnl

Modeling Details

In the iSDG, population is represented as a stock variable, and births, deaths, and net migration as flows. The population stock is disaggregated into 2 genders and 101 age-cohorts by way of subscripts. The representation of the age cohort system and the aging process are conceptually straightforward, a single equation shifts the population in each age cohort (minus cohort deaths and net migration) to the next age cohort at the end of the year, except the cohort over 100. Since the last age cohort does not have a “next” cohort to move to, the population in the last cohort does not age. Also, since the first age cohort does not have a “previous” cohort, it only receives additional people from the births flow.

Footnotes and References

[1] Blue arrows indicate positive effect of a variable on another (i.e. a positive change in a variable leads to a positive change in another variable); red arrows indicate negative effect of a variable on another (i.e. positive change in a variable leads to a negative change on another variable). Unless differently indicated, all diagrams for the remaining sectors follow this coloring code.

[2] United Nations Department of Economic and Social Affairs: Population Dynamics

[3] Nahmias, P. (2004). Fertility behaviour of recent immigrants to Israel: A comparative analysis of immigrants from Ethiopia and the former Soviet Union. Demographic Research, 10, 4, pp. 83-120, Max Planck Institute for Demographic Research Konrad-Zuse Str. 1, D-18057 Rostock, Germany

[4] Dmnl is the standard abbreviation for “dimensionless”, i.e. it refers to a variable such as a % or a normalized indicator.