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".


  • 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.

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