National Records of Scotland

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2004-based Population Projections for Scottish Areas: Uses and Limitations of Projections

2004-based Population Projections for Scottish Areas: Uses and Limitations of Projections

5.1 Population projections have limitations. A projection is a calculation showing what happens if particular assumptions are made. The sub-national population projections are trend-based. They are, therefore, not policy-based forecasts of what the government expects to happen. Many social and economic factors influence population change including policies adopted by both central and local government. The relationships between the various factors are complex and largely unknown. A single projection for each area is given in this publication, but this is not a confident forecast of what will happen and it is not possible to define helpful statistical limits within which the outturn is likely to fall.

5.2 The effect of the assumptions - about future migration, fertility and mortality - is often limited in the sense that there is considerable inertia in population change. The future population of an area is strongly influenced by the initial base population. Because the process of change is cumulative, the reliability of projections decreases over time. Change affects some populations more rapidly and more seriously than others. Thus, projections for areas with small populations tend to be less reliable than those for areas with large populations, because the former are usually affected more by migration. Projections of the number of adults (particularly elderly people) are usually more reliable than those for children because of difficulties in projecting levels of fertility and parental migration. The size of the migration flows, and the uncertainty of future trends, are such that for many areas the migration assumptions are more critical than the fertility and mortality assumptions. Hence the migration assumptions can have a large effect on small populations in the long-term (e.g. the Shetland Islands) and also for some other areas with larger populations (e.g. Aberdeen City).

5.3 Central government population projections set local and regional population patterns into a national context. They are trend-based. However, it should be remembered that new local planning policies are often intended to modify past trends. Structure plans may be based on reasoned and agreed departures from the projections that seem better able to fit particular local circumstances.

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