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

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Developing stochastic population forecasts for the United Kingdom:Progress report and plans for future work

Emma Wright

Office for National Statistics

national population projections
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


  • Results on the ONS website
uncertainty in population projections
Uncertainty in population projections
  • Demographic behaviour is inherently uncertain
  • Any set of projections will inevitably be proved wrong to a greater or lesser extent
principal variant projections
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
ons stochastic forecasting project
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
probability distributions
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

model drivers
Model Drivers
  • Fertility – Total Fertility Rate
  • Mortality – Male and female period life expectancy at birth
  • Migration – Total net migration
deriving probability distributions for the ons model
Deriving probability distributions for the ONS model
  • Expert opinion - NPP expert advisory panel questionnaire
  • Past projection errors - GAD historic projections database
expert opinion
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.
generating drivers
Generating drivers

Overall model

Pt = Pt-1 + Bt – Dt + Mt

Random walk with drift (RWD) model:

Drivert = Drivert-1 + Valuet + Driftt

  • Based on cohort component model
  • UK only
  • Random numbers generated
  • Age distributions
  • 5,000 simulations
  • 2006-2056 projection period
  • 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
future plans
Future plans
  • Use of time series approach
  • Deriving probability distributions
  • The RWD model
  • Correlations
  • Net migration
  • Age Distributions

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