Emma Wright Office for National Statistics

1. Developing stochastic population forecasts for the United Kingdom:Progress report and plans for future work Emma Wright Office for National Statistics

2. National population projections • Dependent on assumptions about future levels of fertility, mortality and migration which are reviewed every two years • Latest projections based on the population at mid-2008 • Results on the ONS website

3. Uncertainty in population projections • Demographic behaviour is inherently uncertain • Any set of projections will inevitably be proved wrong to a greater or lesser extent

4. Past UK population projections

5. Mean projection error by age groupPast UK projections

6. Principal & variant projections • Principal projections - based on assumptions thought to be the best at the time they are adopted • Variant projections – plausible alternative scenarios, NOT upper or lower limits. • Limitation - principal and variant projections are deterministic, no measure of probability

7. Total UK Population2008-based principal and variant projections

8. ONS Stochastic forecasting project • Aim • To develop a model that will enable the degree of uncertainty in UK national population projections to be specified • Approach • Express fertility, mortality and migration assumptions in terms of their assumed probability distributions

9. Probability distributions How can we estimate future probability distributions? Three approaches: • Analysis of past projection errors • Expert opinion • Time series analysis No ‘right’ answer – subjective judgement

10. Model Drivers • Fertility – Total Fertility Rate • Mortality – Male and female period life expectancy at birth • Migration – Total net migration

11. Deriving probability distributions for the ONS model • Expert opinion - NPP expert advisory panel questionnaire • Past projection errors - GAD historic projections database

12. Expert Opinion • National Population Projections Expert Advisory Panel (set up via BSPS): • David Coleman Phil Rees • Mike Murphy Robert Wright • John Salt John Hollis • Expressed opinions on the most likely levels and 67% confidence intervals for TFR, period life expectancy at birth and net migration in 2010 and 2030.

13. Generating drivers Overall model Pt = Pt-1 + Bt – Dt + Mt Random walk with drift (RWD) model: Drivert = Drivert-1 + Valuet + Driftt

14. UK TFRPrediction intervals v 2006-based assumptions

15. UK male period life expectancy at birth Prediction intervals v 2006-based assumptions

16. UK net migration Prediction intervals v 2006-based assumptions

17. Program • Based on cohort component model • UK only • Random numbers generated • Age distributions • 5,000 simulations • 2006-2056 projection period

18. Provisional results UK age structure 2031

19. Provisional resultsUK age structure 2056

20. Limitations • Do not know true probability distributions • Validity of results wholly dependent on assumptions underlying model • Inflated sense of precision • Communicating results and limitations may be a challenge • BUT….if aware of the limitations, then stochastic forecasting can be a useful approach

21. Future plans • Use of time series approach • Deriving probability distributions • The RWD model • Correlations • Net migration • Age Distributions

22. Questions? If you would like to feed in any comments on this work, please e-mail: natpopproj@ons.gov.uk