developing stochastic population forecasts for the united kingdom: progress report and plans for...
TRANSCRIPT
Developing stochastic population forecasts for the United Kingdom:Progress report and plans for future work
Emma Wright
Office for National Statistics
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
Uncertainty in population projections
• Demographic behaviour is inherently uncertain
• Any set of projections will inevitably be proved wrong to a greater or lesser extent
Past UK population projections
50
55
60
65
70
75
80
1966 1976 1986 1996 2006 2016 2026 2036 2046 2056
Year
Milli
ons
Actual 1971-based 1977-based 1989-based
1998-based 2004-based 2006-based
Mean projection error by age groupPast UK projections
-30%
-20%
-10%
0%
10%
20%
30%
0-4
5-9
10-1
4
15-1
9
20-2
4
25-2
9
30-3
4
35-3
9
40-4
4
45-4
9
50-5
4
55-5
9
60-6
4
65-6
9
70-7
4
75-7
9
80-8
4
85+
Age group
% E
rro
r
-30%
-20%
-10%
0%
10%
20%
30%
5 years ahead 10 years ahead 25 years ahead
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
Total UK Population2008-based principal and variant projections
50
55
60
65
70
75
80
85
90
95
100
105
110
115
1981 2001 2021 2041 2061 2081Year
Millio
ns
P
HF
LF
LMLL
HLHM
LP
HPProjected
ONS Stochastic forecasting project
• AimTo develop a model that will enable the degree of uncertainty in UK national population projections to be specified
• ApproachExpress fertility, mortality and migration assumptions in terms of their assumed 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
• Fertility – Total Fertility Rate
• Mortality – Male and female period life expectancy at birth
• Migration – Total net migration
Deriving probability distributions for the ONS model
• Expert opinion - NPP expert advisory panel questionnaire
• Past projection errors - GAD historic projections database
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
Overall model
Pt = Pt-1 + Bt – Dt + Mt
Random walk with drift (RWD) model:
Drivert = Drivert-1 + Valuet + Driftt
UK TFRPrediction intervals v 2006-based assumptions
1.0
1.5
2.0
2.5
1971 1976 1981 1986 1991 1996 2001 2006 2011 2016 2021 2026 2031
Year
TFR
Median
Prediction Interval 67%
Actual and PrincipalNPP
Variant NPP
Expert Average 67% CI
projections
UK male period life expectancy at birth Prediction intervals v 2006-based assumptions
65
70
75
80
85
90
1971 1976 1981 1986 1991 1996 2001 2006 2011 2016 2021 2026 2031
Year
EOLBMales(years)
Median
Prediction Interval 67%
Actual and PrincipalNPP
Variant NPP
Expert Average 67% CI
projections
UK net migration Prediction intervals v 2006-based assumptions
-200
-100
0
100
200
300
400
500
1971 1976 1981 1986 1991 1996 2001 2006 2011 2016 2021 2026 2031
Year
Net Migration
(thousands)
Median
Prediction Interval 67%
Actual and PrincipalNPP
Variant NPP
Expert Average 67% CI
projections
Program
• Based on cohort component model
• UK only
• Random numbers generated
• Age distributions
• 5,000 simulations
• 2006-2056 projection period
Provisional results UK age structure 2031
800 600 400 200 0 200 400 600 800
0
10
20
30
40
50
60
70
80
90
100
110
120
Ag
e
(ye
ars
)
Population (thousands)
95%predictiveinterval
67%predictiveinterval
Median
Males Females
Provisional resultsUK age structure 2056
800 600 400 200 0 200 400 600 800
0
10
20
30
40
50
60
70
80
90
100
110
120
Ag
e (
year
s)
Population (thousands)
95%predictiveinterval
67%predictiveinterval
Median
Males Females
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
Future plans
• Use of time series approach
• Deriving probability distributions
• The RWD model
• Correlations
• Net migration
• Age Distributions
Questions?
If you would like to feed in any comments on this work, please e-mail: