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Subnational Population Projections Accuracy and Uncertainty

Centre for Demography. Subnational Population Projections Accuracy and Uncertainty. Presentation to BSPS conference 10 -12 September 2008. 2006 based Projections . Subnational Population Projections (SNPPs) were published in June ‘Southampton won 4 – 2,

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Subnational Population Projections Accuracy and Uncertainty

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  1. Centre for Demography Subnational Population ProjectionsAccuracy and Uncertainty Presentation to BSPS conference 10 -12 September 2008

  2. 2006 based Projections • Subnational Population Projections (SNPPs) were published in June ‘Southampton won 4 – 2, this means that they got exactly the same score as they did against the same opponents last year, when they won 3 – 1.’ A major television sports presenter, on live TV.

  3. Scale of revisions to projections • SNPPs are scaled to national projections, and the 06-based national projection for 2011 was revised upwards by 1.4% (since the previous 2004-based projection) • Nearly 90% of LAs had SNPP revisions within 3% of the national revision (so between 1.6% down and 4.4% up) • 20 LAs on either side of the 3% cut-off, indicating significant revision to their projection that was not due to the revised national constraint

  4. Revisions made to LA projections for 2011 between 04-based and 06-based SNPPs

  5. Largest upward revisions • Top five biggest upward revisions, 2011: • Exeter (9.2%) • Durham (8.2%) • Forest Heath (7.1%) • Charnwood (7.0%) • Southwark (6.9%) • In all cases the mid-year estimates (MYEs) for 2005 and 2006 were above previous trend • Noticeable that top three areas include two universities and an area with a major US military base, all of which make accurate projections more difficult

  6. Largest downward revisions • Top five biggest downward revisions, 2011: • Kensington & Chelsea (6.6%) • Westminster (4.3%) • Newham (4.1%) • West Wiltshire (3.1%) • Fylde (2.7%) • In each LA a previously strong trend of growth tapered off in the 2005 and 2006 MYEs • Top three areas are all in Inner London, where high population mobility makes accurate projections more difficult

  7. Accuracy of Projections:Comparing projections to reality "As far as the laws of mathematics refer to reality, they are not certain; and as far as they are certain, they do not refer to reality.“ Albert Einstein

  8. Accuracy of ProjectionsKey findings • Assessing accuracy is complicated • Changes to base (revisions etc.) • National Projections change • For most areas projections provide a good general purpose tool to indicate likely future population • Projections based on current trends • Smaller geographies less accurate • Further ahead the projection the less accurate

  9. Assessing Accuracy and RMSE • Accuracy assessed by comparing projections to annual mid-year estimates • Mid-year estimates are also subject to statistical error • Root Mean Square Error (RMSE) provides a useful summary measure of difference • RMSE gives a single summary statistic • Gives a figure for the average difference a ‘geography’ will have • Allows approximation of a confidence interval

  10. Historical Accuracy

  11. Accuracy at Local Authority level • Accuracy three years ahead useful indicator • RMSE at LA district level is 1.4% • So expect 95% of LAs to be within +/- 2.8% • Areas of high migration most difficult • Student areas can be difficult • And some armed forces areas (but not all) • Full report, including supporting data available on website • www.statistics.gov.uk/snpp

  12. Uncertainty In this world nothing can be said to be certain, except death and taxes. (Benjamin Franklin, 1789, after Defoe)

  13. Projection Variants • Consulted on possibility of producing projection variants in the spring • 48 separate responses • Prompted a range of opinions

  14. Two Consultation Response Extracts • Variant projections would be very useful in getting policy-makers to more fully appreciate the importance of considering potentially different future scenarios. These would hopefully help to enable better planning of future services. • Publication of variant data sets to a lower than national / regional level could be misleading; whilst valid in their own right, their publication away from the principal projection has the potential to ‘muddy’ the water and generate misleading interpretations. These were comments to a consultation and do not represent the views of ONS.

  15. Projection Variants • Consulted on possibility of producing projection variants in the spring • 48 separate responses • Prompted a range of opinions • Will intend to publish a full report on 29 September • Reporting here some headline conclusions

  16. Use and Demand • Main reasons quoted for needing variants • Geospatial planning • Planning for service provision • To illustrate uncertainty • To allow scenario planning • Large majority of respondents indicated that producing variants would be valuable • Conclusion: there is a strong case for producing subnational variants, subject to resources

  17. A Housing Variant • Single most cited option • Very strong demand … • but not universal.

  18. Two Consultation Response Extracts: Housing Variant • I do not believe that a proper housing capacity variant can be produced without local assumptions about vacancy, sharing, representative rates, etc, etc. If an independent attempt is made to project population using migration levels that are believed to be ‘consistent’ with planned development it is almost certainly a waste of time. • The housing scenario variant is of vital importance to Barnet because the council is actively supporting the increase of the borough’s residential stock by 25 % in the next 20 years. We would compare ONS figures with the GLAs products to better understand the effect of different assumptions.

  19. A Housing Variant • Single most cited option • Very strong demand … • but not universal. • Some experience (especially Chelmer, POPGROUP/HOUSEGROUP, and GLA) • General agreement that regional spatial strategies provide sufficient data • but some concerns. • Difficulties acknowledged by some • but a strong desire to see these overcome. • Conclusion: very strong demand and a strong case made, so ONS intend to examine feasibility further • but due to complexity work is likely to be on a slower path than standard variants.

  20. Standard Variants • Strong Demand • Variants most prioritised • High and Low Population • High and Low Migration • Zero migration (or similar) • Many others mentioned • But no consensus as to which is next important • Conclusion: ONS intend to pursue production of five variants listed above, resources permitting

  21. Other Issues • Clear that migration research has to remain overall priority for ONSCD • Very strong guidance needed on variants • Usage guidance • Status • Implications • Methods and Assumption

  22. And Finally … • Projections provide a useful planning tool, but due account needs to be taken of uncertainty. If you fail to plan, then you plan to fail. (Source unknown)

  23. Contacts • Subnational Population Projections • Room 2300 • ONS • Segensworth Road • Fareham • Hants. • PO15 5RR • 01329 444669 • snpp@ons.gsi.gov.uk • www.statistics.gov.uk/snpp

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