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 group Past 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 Population 2008-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 ColemanPhil Rees Mike MurphyRobert Wright John SaltJohn 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 P t = P t-1 + B t – D t + M t Random walk with drift (RWD) model: Driver t = Driver t-1 + Value t + Drift t

14. UK TFR Prediction 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 results UK 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