The Cohort Survival Model (also called the Cohort-Component Method) is the most widely used method for population projections. It projects the future size and composition of a population by following age-sex groups (cohorts) through time and applying assumptions about fertility, mortality, and migration.

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Steps in the Cohort Survival Model

1. Divide the population by age and sex (e.g., 0–4, 5–9, 10–14, …).
2. Apply survival ratios (Sx) to each cohort, based on mortality rates (from life tables), to estimate how many survive to the next age group.
   • Example: If 100,000 children aged 0–4 have a survival ratio of 0.95, then 95,000 will survive to the 5–9 group.
3. Add new births by applying age-specific fertility rates (ASFRs) to women of reproductive ages (15–49). These births form the new 0–4 age cohort.
4. Adjust for migration (in-migration and out-migration) if applicable.
5. Repeat the process for each projection interval (usually 5 or 10 years).

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Example (Simplified)

• Population in 2011: 1,00,000 children in age group 0–4.
• Survival ratio from 0–4 → 5–9 = 0.95.
• Projected survivors in 2016 (age 5–9) = 95,000.

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Inter-Regional Cohort Survival Model

Definition

The Inter-Regional Cohort Survival Model is an extension of the cohort survival model that incorporates migration between regions. Instead of treating the population as a whole, it simultaneously projects multiple regions and distributes people across them according to migration flows.

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Steps in Inter-Regional Model

1. Divide the population by age, sex, and region (e.g., Region A, Region B, Region C).
2. Apply survival ratios (mortality) within each region.
3. Estimate migration flows between regions using a migration matrix:
   • Shows how many people of each age/sex group move from one region to another.
   • Example: 5% of 20–24-year-olds in Region A migrate to Region B in the next 5 years.
4. Add fertility contributions (births) in each region, based on the number of women and regional fertility rates.
5. Sum up to obtain future age-sex-region-specific population.

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Uses

• Cohort Survival Model: National population projections (fertility, mortality, migration considered as aggregates).
• Inter-Regional Model: Regional/urban planning, migration studies, distribution of schools, hospitals, housing, transport needs.

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Key Difference

Feature	Cohort Survival Model	Inter-Regional Cohort Survival Model
Scope	Entire population (national level)	Multiple regions simultaneously
Migration Treatment	Net migration added/subtracted	Explicit inter-regional flows (origin–destination matrix)
Usefulness	National projections	Regional/urban planning, migration analysis

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Conclusion

• The Cohort Survival Model is the foundation of demographic projection, focusing on fertility, mortality, and net migration.
• The Inter-Regional Cohort Survival Model refines this by including detailed migration between regions, making it essential for regional planning and policy.