# Probabilistic Projection of Subnational Total Fertility Rates

## TL;DR

This paper develops a probabilistic method for projecting subnational total fertility rates that aligns with UN standards and demonstrates superior predictive performance across diverse countries.

## Contribution

It introduces a region-specific scaling method for TFR projections that improves accuracy and supports the Watkins hypothesis of within-country convergence over time.

## Key findings

- The scaling method outperforms other approaches in predictive accuracy.
- It effectively captures within-country TFR correlation.
- Supports the Watkins hypothesis over the last 50 years.

## Abstract

We consider the problem of probabilistic projection of the total fertility rate (TFR) for subnational regions. We seek a method that is consistent with the UN's recently adopted Bayesian method for probabilistic TFR projections for all countries, and works well for all countries. We assess various possible methods using subnational TFR data for 47 countries. We find that the method that performs best in terms of out-of-sample predictive performance and also in terms of reproducing the within-country correlation in TFR is a method that scales the national trajectory by a region-specific scale factor that is allowed to vary slowly over time. This supports the hypothesis of Watkins (1990, 1991) that within-country TFR converges over time in response to country-specific factors, and extends the Watkins hypothesis to the last 50 years and to a much wider range of countries around the world.