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.
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