Document and verify source of long-run economic projections (2031+)

[MaxGhenis]

Summary

The yoy_growth.yaml file contains economic projections extending to 2073, but the sources for values beyond the OBR forecast horizon (~2030) are not clearly documented or verified.

Current State

• 2024-2030: Values come from OBR EFO detailed forecast tables (well documented)
• 2031+: Values appear to be based on OBR long-run equilibrium assumptions, but:
  • The specific year-by-year values (e.g., RPI 2.30% in 2031, gradually rising to 2.39%) are not found in published OBR tables
  • It's unclear if these were interpolated, calculated from methodology, or sourced from supplementary tables

Affected Parameters

• obr.rpi (2031-2073)
• obr.average_earnings (2031-2073)
• obr.consumer_price_index (2031-2073)
• obr.cpih (2031-2073)

Questions to Resolve

1. Where did the 2031+ values originally come from?

   • OBR "Fiscal risks and long-term projections" publication?
   • OBR supplementary long-term economic determinants tables?
   • Custom calculation based on OBR methodology?

2. Should we update these when new long-term projections are published?

3. If calculated, should we document the methodology in the file or link to code that generates them?

Related

• PR Add OBR long-term economic determinants #1305 added these long-term values but didn't document the source
• The header comment in yoy_growth.yaml references OBR methodology but not specific tables

Potential Sources to Check

• OBR Fiscal risks and long-term projections (July 2025)
• OBR Long-term economic determinants supplementary tables
• OBR long-run RPI-CPI wedge assumptions