Uses and Limitations of Population Projections
Uses and Limitations of Population Projections
It is increasingly important to have high quality statistics on the population and projections of the population, for policy development and for planning and providing public services in different geographic areas. They are used for:
- central and local finance allocation;
- informing the provision of nurseries or day care centres;
- informing local and national policy;
- housing and land use planning;
- health care planning;
- modeling and projecting health care indicators;
- weighting surveys;
- benchmarking other projections and as a control for smaller area projections;
- teacher workforce models both at a national and local level;
- looking at the implications of an ageing population;and
- making national and international comparisons, etc.
But population projections have limitations. A projection is a calculation showing what happens if particular assumptions are made. The 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.
The effect of the assumptions about future migration, fertility and mortality is often limited by the inertia in population change, the future population of an area is strongly influenced by the initial base population. As 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, mean 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 where there is a small population) and also for some other areas with larger populations (e.g. East Dunbartonshire).
Central government population projections set local and regional projected population patterns in 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 placed to fit particular local circumstances.
Consequences of projections
Population projections, like some other types of projections, may indicate that existing trends and policies are likely to lead to outcomes which are judged undesirable. If new policies are then introduced, they may result in the original projections not being realised. However, this means the projections will have fulfilled one of their prime functions, to show the consequences of present demographic trends with sufficient notice for any necessary action to be taken.
Variant projections are produced to give users further information on the projected populations of Scottish areas, and to give an indication of the inherent uncertainty of demographic behaviour, especially for long-term projections. The variants allow users to consider the impact upon the demand and supply of services, such as education, health, and care of the elderly, if future fertility, mortality and migration differ from the assumptions made for the principal projections. They are not intended to represent upper or lower limits but to illustrate plausible alternative scenarios of future demographic behaviour.
Accuracy of projections
Past projections were compared with the mid-year estimates as part of the review for the Population Projections Working Group (PPWG) into the sub-national projections for administrative areas produced by the National Records of Scotland (NRS). The paper, PPWG (06) 09 - Accuracy of the Sub-National Population Projections for Scotland (available on this website) looking at the accuracy of the projections was published in 2006. The other papers from this group are also on this website. There is also information on the change between the most recent projection and the previous set in the publications themselves.
It should be noted that as these population projections are trend based, they are less reliable in periods of rapid change. For example, the recent change in volume of migrants from the A8 accession countries to Scotland was not picked up by earlier projections.