1. What would we like to know about the future
What would we like to know about the future? Alternative approaches to sub-national population projections
Philip Rees1, Nik Lomax1, Paul Norman1, Pia Wohland2 and Stephen Clark3
1School of Geography, University of Leeds
2Hull-York Medical School
3Leeds Institute for Data Analytics
BSPS Day Meeting, Monday 27 March 2017, University of Leeds
UK Variant sub-national population projections and population projections by ethnicity

2. Topics to be covered Types of projection: variants to constraints
Projections for sub-populations: methods
Ethnic group projections for Britain
Adding new features to projections

3. Types of Projection
Variant projections: highs and lows for component assumptions to measure possible ranges
Scenario projections: variants linked to policy/events
Probabilistic projections: ensemble projections to measuring uncertainty/confidence
Reference projections: counter-factual projections to measure impact, usually of international migration
Decomposition projections: schemes for measuring the impact of component assumptions together
Sensitivity projections: tests of the effect of decisions on models, sub-populations or assumptions
Constrained projections: “One Ring to Rule Them All” , fitting sub-national projections to national projections

4. Official National Population Variants ONS (2016) National Population Projections: 2014-based Statistical Bulletin. Figure 2: Estimated and projected population of the United Kingdom, mid-2001 to mid
Problem: Does this range between high and low represent a given probability range (e.g. 10/80/10 or 5/90/5). In 2012 Lord Filkin asked me. Can you estimate a probability that in 25 years the old age population will reach a particular share? House of Lords (2013) Ready for Ageing? Report. Select Committee on Public Service and Demographic Change. The Stationery Office, London.

5. Mortality variants: should we consider lower life expectancy?
Has the long-term downward trend in mortality been checked by austerity?
Hiam L, Dorling D, Harrison D & McKee M (2017) Why has mortality in England and Wales been increasing? An iterative demographic analysis. Journal of the Royal Society of Medicine,
Do we need a life expectancy decrease scenario?

6. Alternative BREXIT International Migration Scenarios
Long-term limit assumed from ,
NewETHPOP projections
Long-term limit assumed from , projections for Thames Water

7. How should internal migration variants be designed?
Patterns
: GFC Recession Year: less out-migration from London
: Boom Year: more out-migration from London
: Big Boom Year: most out-migration from London
Scenarios:
1 year ( ): less out-migration from London
5 year: ( ): moderate out-migration from London
10 year ( ): most out-migration from London
Figure 3.2: Change in internal net migration rates, , and
Source: Lomax, N. and Stillwell, J. (2018) Temporal Change in Internal Migration in the United Kingdom. Chapter 6 in: Champion, T., Shuttleworth, I. and Cooke, T. Internal Migration in the Developed World: Are We Becoming Less Mobile? Routledge

8. Reference projections to measure the impact of international migration assumptions
No International Migration Population as % of With International Migration Population in 2061:
Total Population: ~75%
White British: ~90%
White Other: ~27%
Chinese: ~22%
Source: Philip Rees, Pia Wohland, Stephen Clark, Nik Lomax and Paul Norman (2016) The Future is Diversity: New Forecasts for the UK’s Ethnic Groups. Paper presented at the European Population Conference 2016, Johannes Gutenberg Universität, 31 August – 3 September

9. Probabilistic projections: What the UN says about the UK’s future population
Problem: Probability distributions for fertility and mortality drivers are estimated and sampled but international migration is handled deterministically using net international migration trajectories that assume convergence to zero. Pie in the sky!?

10. Scenario Projections: Wittgenstein Centre’s Forecasts for the UK Population
Problem: Are these SSPs really relevant for the UK? International migration is simply handled as a constant set of out-migration and in-migration rates in a bi-regional model. From 2060 to 2100 these rates are assumed to converge ending up with net balance of zero. Pie in the sky!? We need a multi-regional model as one country’s emigrants are another country’s immigrants.

11. Sub-national projections: does ethnic disaggregation matter?
The two projections use the same UK overall assumptions
Results the same for international migration
Results about the same for deaths
Results much higher for births (and hence population after 2031)
Why? Because NewETHPOP uses ethnic group sub-populations and ethnic specific fertility rates
Comparison of NPP 2014 Principal (red dots) and NewETHPOP ONS aligned projections (blue dots) for the UK, 2011 to 2061

12. Sub-populations: our NewETHPOP methods
Status: academic not official, well but not fully tested
Ethnicity: harmonised ethnic groups
Geography: harmonised LADs for UK
Ingredients:
Estimated ethnic fertility, mortality, internal migration, international migration
Added processes: mother-child ethnicity; considered changing ethnic identity but did not implement; added special populations but did not implement
Sacrificed the internal migration detail of a multi-regional model by using a bi-regional model to cope with ethnic detail (4,668 sub-populations by SYA)
Evaluated how well we had done (“could do better”)
Therefore built time series of components reconciled to 2001 and 2011 censuses; used 2011 CBBE rates and flows as baseline for projection; did not extend ethnic estimates beyond 2011 because information poor (APS)
Worried about the form of the model: transition versus movement approach (early projections with the former, later projections with the latter)
Used R routines extensively in the estimation and in the projections
Full data deposit of two projections (LEEDS L2 ONS Aligned, LEEDS L1 Brexit-V2) at the UK Data Archive. See ethpop.org for our publications and presentations. Database to come.

13. The Ethnic Transition: trends in 12 groups
2001 Based Projection
Plotted on the graphs are the projected populations expressed as percentages of the 2011 populations
2011 Based Projection
Both projections forecast large increases in 10 of 11 ethnic minority populations, with either no change or small decreases in the White British & Irish and Black Caribbean populations. We call this the ethnic transition.

14. The changing diversity of England’s LAD populations: diffusion of the ethnic transition
2001 Based, MY2011
2001 Based, MY2051
2011 Based, MY2011
2011 Based, MY2051

15. Ethnic population projections: validation
GLA versus Leeds ethnic population forecasts: Asian composition of London Borough populations Asian = Indian + Pakistani + Bangladeshi

16. Decomposition projections: used to assess the impact of component assumptions
Rees, P., Wohland, P. and Norman, P. (2012) The demographic drivers of future ethnic group populations for UK local areas 2001–2051. Geographical Journal, 179(1): DOI: /j x

17. prevalence rate methods used to project health
Sub-populations:
prevalence rate methods used to project health
“Drawing on a range of projections, Professor Rees found that population ageing will increase the population suffering from limiting long-standing illness by 39% between 2010 and 2050, but that if the decreasing trends of the last decade are reproduced in the next four decades, the increase will be clawed back to 6%.”
Ready for Ageing? Paragraph 74
Rees, P., Chengchao, Z., Wohland, P., Jagger, C., Norman, P., Boden, P., Jasinska, M. (2013) The implications of ageing and migration for the future population, health, labour force and households of Northern England. Applied Spatial Analysis and Policy, 6(2): DOI: /s

18. Conclusions
There is a large range of population projection methods and outcomes that can inform national and local planning.
Variants are the first step in assessing those outcomes
National fertility, mortality and international variants can be converted into local variants
Local internal migration variants need more consideration as they are linked together
It would be easy to design variants that extend current reference projections into a range of decomposition variants
Sub-national projections are currently constrained to national and the adjustments are small. This does not appear to the case if local ethnic group projections are run. One ring does not rule them all.
The case for official ethnic projections needs to be made. Our projections are being used, for example, by Police Authorities and the Army to align recruitment to the ethnic composition of young people. Our projections are used as inputs to analyses of specific morbidities. But is this enough given public expenditure constraints?
Scenarios go beyond variants in helping evaluate the effects of events and policy but must be tailored to UK local issues.
Probabilistic fan diagrams of future population are attractive but so far migration has been the methodological Cinderella. Is there a fairy godmother out there?