National Records of Scotland

Preserving the past, Recording the present, Informing the future

Discussion: Local Projections - Top Down or Bottom Up?

Discussion: Local Projections - Top Down or Bottom Up?

28 November 2005

Clive Lewis, Statistical Directorate, Welsh Assembly Government

Projections are not forecasts; they are:

"simply 'scenarios' (the certain, computational outcome of a given set of assumptions), rather than forecasts of the most likely course of future events".

Needs:

  • Increasing interest in using them to allocate resources and assess service needs at a local (authority) level;
  • Private sector organisation interest in future, localised developments;
  • Need to set policies or strategies in a demographic context and to assess impacts on local populations.

How best to meet them:

  • No single method suitable for all purposes;
  • 'Disaggregating' the GAD national projections i.e. the top down approach, of limited application because:
    • though commonly referred to as ‘trend based’; this is misleading. They are a hybrid;
    • they are a means of allocating the nationally projected figures according to recent, relative sub-national demographic features;
    • for the short term, they are simply a means of moving from the base year to when the long-term assumptions kick-in;
  • Need for sub-national population projections that:
    • reflect recent, local trends;
    • take account of local factors that will be key in influencing future population growth or influence the demographic profile of the local population.

The bottom-up approach:

  • Made at the local (authority) level, unconstrained to the national projections:
    • true, trend-based projections for each authority area, produced in a consistent way using a common model;
    • policy-based projections generated by each local authority as part of its development plans – effectively forecasts – with the assumptions reflecting the authority’s policy goals.

This presentation is available in Adobe Acrobat Portable Document Format (PDF)
PDF Format (10 Kb).

PDF files require Acrobat Reader. Download Acrobat Reader free Get Acrobat Reader