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2004-based Population Projections for Scottish Areas: Methodology and Assumptions

2004-based Population Projections for Scottish Areas: Methodology and Assumptions

4.1 Methodology

4.1.1 The results are produced by the demographic component method using a single year projection model (see paragraph 4.1.2). That is, a projection is made by sex and single year of age (up to age 90 & over) for each future year. This is done for arithmetical convenience and should not be taken to imply that reliable projections can be made in such detail.  However, it provides 'building blocks' which users can aggregate into age-groups of their choice for the years in which they are interested. For example, the production of results in standard five-year age groups (0-4, 5-9, 10-14, etc.) does not help the education planner wishing to know the size of the future population of secondary school age. Also, as some planning is done on a 'rolling' basis (for example 10 years from a moving base date), it is useful to be able to provide a projection for any future year rather than just selected years.

4.1.2 The projection starts with the population estimates for the base year, disaggregated by single year of age, sex, and area. This base population is then projected one year ahead. First, an estimate of the numbers surviving to be one year older is made by applying a series of mortality rates to give the numbers of deaths, and hence survivors, at each age. The numbers of live births in the year are produced, using fertility rates in combination with the female populations of child bearing age; and an allowance is made for infant mortality. Lastly, the expected number and age/sex structure of people entering and leaving the area is taken into account in order to cover changes in the population due to migration. These three components of population change, together with the starting population, combine to form a projection of the population one year from the base date. The process can be repeated as often as required. For each year of the projection period, it is necessary to make assumptions about the future fertility rates (to give the number of births), mortality rates (for deaths) and migration. The following paragraphs describe the base population, the small changes made to the software and the method compared with previous projections as well as the assumptions made in the new projections.

4.2 Base population

4.2.1 The Registrar General's 'Mid-2004 Population Estimates, Council and Health Board Areas', published in April 2005 and revised in July 2007 on this website, were used as the base population. These cover all persons usually resident in each area, whatever their nationality. Usual residents temporarily away from home are included, but visitors are excluded. Students are taken to be resident at their term-time address. Members of HM and non-UK Armed Forces stationed in Scotland are included; HM forces stationed outside Scotland are excluded. Population figures relate to 30 June 2004 and ages relate to age last birthday. More information on the 'Revised Mid-year Population Estimates 2003-2006' can be found on this website.

4.3 Projections software system and other small method changes

4.3.1 The software used to produce the population projections is an in-house MS Excel macro system which has been developed over a number of years. The system runs on 36 'building bricks' – areas which aggregate to both council and NHS board areas. The availability of small area population estimates for data zones has meant that the input data for the system can now be aggregated from these estimates. However, as data zones do not nest exactly in to NHS board areas (they are only 'best fit'), small adjustments have been made to the projection results based on the difference between the aggregated NHS board populations from data zones and those estimated on the correct boundaries for the mid-year population estimates. These small adjustments mean that the projections presented in this paper for NHS board areas are very close to the correct boundaries.

4.4 Fertility

4.4.1 The projected number of births was obtained by applying age specific fertility [Footnote 1] rates to the numbers of women at each childbearing age, for each year of the projection period. In determining the fertility rates used in the national projections for Scotland, assumptions were made about the average completed family size for successive generations of women. This measure tends to be more stable over time than fertility rates for specific years (so called period fertility rates), because of generational differences in the timing of having families. It was assumed that the average completed family size will continue to decline from around 1.90 children per woman for those born in the late 1950s and now reaching the end of their childbearing lives, before levelling off at 1.60 for those born in the 1990s and later. A downward trend in the number of births is expected, to a projected level of about 49,000 by 2024. The underlying assumptions of average completed family size for the current projections are shown in the table below. The resultant age specific fertility rates assumed for Scotland as a whole are given in Annex A. More information on the fertility assumptions for Scotland can be found in Annex A to the publication 'Projected Population of Scotland (2004-based)' on this website.

4.4.2 The projected number of births by administrative area is shown in Table 4. The percentage change in the number of projected births between 2004 and 2024 by council area is also shown in Figure 10.   

4.4.3 For local areas, the assumed national fertility rates have been adjusted to take account of local variations observed in the five year period preceding the projection, as recommended by the Population and Migration Statistics Committee (PAMS) rather than the three year period used in previous projection rounds. The local scaling factors used to adjust the national rates are given in Annex C.

4.5 Mortality

4.5.1 The projected number of deaths each year was calculated by applying mortality rates by age and sex to the appropriate sub-populations. The national rates for the first year of the projections, 2003-04, were based on autumn 2005 estimates of the numbers of deaths at each age in that period. The mortality rates for later years were based on long-term trends up to 2003-04. Future improvements in mortality rates are based on the trend observed in the period 1961 to 2003. It is assumed that annual rates of reduction in mortality rates will tend toward a common reduction at each age of 1 percent a year by 2029. Thereafter, the mortality improvement is assumed to continue at this rate (in contrast to the previous projections where it was assumed to halve every subsequent twenty-five years). In line with long-term trends, it has been assumed that the mortality rates for Scotland will continue to be higher at most ages than those for England and Wales.

4.5.2 Based on these rates, expectation of life at birth is projected to increase from 74.3 in 2003-04 to 78.3 in 2023-24 for males; and from 79.4 in 2003-04 to 82.8 in 2023-24 for females. The national mortality rates are shown, for selected ages and for selected years of the projection, in Annex B.

4.5.3 Similar to the fertility assumptions for local areas, the assumed national mortality rates have been adjusted to take account of local variations observed in the five year period (rather than the three year period)  preceding the projections. The local scaling factors used to adjust the national rates are shown in Annex C.  More information on the mortality assumptions for Scotland can be found in Annex B to the publication "Projected Population of Scotland (2004-based)" on this website.

4.6 Migration

4.6.1 Assumptions about future levels of migration to and from Scotland were based on analysis of trends in civilian migration to and from the UK and between the four constituent countries of the UK. Reflecting the most recent information, the latest projections assume net in-migration of 4,000 from 2007-08 onwards; the previous sub-national projections, assumed net out-migration of -1,500 per year for the long-term. In the first three years of the new projection higher net inflows are assumed, reflecting these recent trends. It is assumed that in the short-term there will be a net inflow of 21,000 migrants to Scotland in 2004-2005, 13,500 in 2005-2006 and 8,500 in 2006-2007 before the level drops to an assumed net inflow of 4,000 for the rest of the projection period. This reflects recent increases in the number of people migrating to Scotland, after many years when net out-migration was the norm. More information on the migration assumptions for Scotland can be found in Annex C to the publication 'Projected Population of Scotland (2004-based)' on this website. 

4.6.2 The net migration assumptions for local areas were made after consultation with other central government departments and local authorities. They are based on an analysis of recent trends in migration. The assumptions for each area are shown in Annex D. It is important to remember that the local assumptions have to match the totals used in the national projections and that for the longer term, migration assumptions are highly speculative. The trends in net migration for the previous five years are shown in Annex E.

4.6.3 Council and NHS board area specific age/sex distributions have been assumed for the in- and out- migrant flows using information from the National Health Service Central Register (NHSCR) – on movements of patients between the lists of GPs - observed in the previous three years and 2001 Census migration data flows. These distributions have been made consistent with the age/sex distribution used for Scotland in the national projection.

Footnote

1. Fertility in this paper refers to the demographic number of children that would be born rather than the common usage of the term referring to biological fertility.

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