National Records of Scotland

Preserving the past, Recording the present, Informing the future

Methodology and Assumptions

Methodology and Assumptions

Methodology

The results are produced by the demographic component method using a single year projection model (see next paragraph). That is, a projection is made by single year of age (up to age 90 & over) and sex 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 as well as the 'traditional' years i.e. census years (e.g. 2001) and the fifth year after a census (e.g. 2006).

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 and the assumptions made.

Base population

The Registrar General's mid-2000 population estimates, were used as the base population. See 'Population Estimates 2000 (publication)' on this website. 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.

Fertility

The projected number of births was obtained by applying fertility 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, because of generational differences in the timing of having families. It was assumed that the average completed family size would continue to decline from just under 2 children per woman for those born in the mid to late 1950s and now reaching the end of their child bearing lives, to 1.59 for those born in the 1980s, before rising again slightly to 1.60 for those born in 1990 and later. This is considerably lower than the long-term rate of 1.75 which was assumed in the 1998-based projections and which is the long term rate assumed for England and Wales in the 2000-based projections. Births in recent years have fallen well below the previously projected level and birth rates for Scotland have been consistently lower than for the rest of the UK. There would need to be a very strong rise in the number of births to women over 30 for the average completed family size of women born in 1975 to rise above 1.60 and there is no evidence to suggest that women born in 1980 and later will have more children. The underlying assumptions on average completed family size for the current projections are shown in the table directly below. The resultant age specific fertility rates assumed for Scotland as a whole are given in Appendix A.

 Average completed family size assumptions

Women born at the beginning of calendar year

Assumed average completed family size

Average family size completed by mid-2000

Average age at motherhood (years)

1950

2.08

2.08

26.1

1955

1.95

1.95

26.7

1960

1.87

1.84

27.3

1965

1.78

1.56

28.0

1970

1.67

1.00

28.4

1975

1.60

0.46

28.8

1980

1.59

0.15

28.9

1985

1.59

-

28.9

1990

1.60

-

28.9

1995

1.60

-

28.9

2000 & later

1.60

-

28.9

Projected births and the difference from 1998-based projections are shown in the table below. The annual number of births is expected to fall from around 53,000 in 2000-01 to around 50,000 by the end of the projection period.

 Projected births and the difference from 1998-based projections

(thousands)

Year

Live births

Difference from 1998-based

2000-2001

53

-3.9

2001-2002

52

-4.2

2002-2003

52

-3.9

2003-2004

52

-3.7

2004-2005

51

-3.8

2005-2006

51

-3.9

2006-2007

51

-4.1

2007-2008

51

-4.3

2008-2009

50

-4.4

2009-2010

50

-4.6

2010-2011

50

-4.8

2011-2012

50

-4.9

2012-2013

50

-5.0

2013-2014

50

-4.9

2014-2015

50

-4.8

2015-2016

50

-4.7

For local areas, the assumed national fertility rates have been adjusted to take account of local variations observed in the three year period preceding the projection. The local scaling factors used to adjust the national rates are given in Appendix C. Projected births by area are shown in Table 4.

Mortality

The projected number of deaths each year was calculated by applying mortality rates by age and sex to the appropriate sub-populations. The Government Actuary's Department based the national rates for the first year of the projections, 2000-01, on estimates of the numbers of deaths in that period that were available in the Autumn of 2001. The mortality rates for later years were based on long-term trends up to 2000-01. Generally, mortality rates were assumed to fall, though the rates for males in their mid-twenties to mid-thirties and their mid-forties are assumed to increase slightly in the early years of the projections. In line with the long-term trends, it was assumed that the mortality rates for Scotland would continue to be higher at most ages than those for England & Wales.

Overall, the mortality rates assumed for these projections are similar to those that were used in preparing the 1998-based projections. The assumed expectation of life in 2016 has decreased slightly for females but more markedly for males, compared with the 1998-based projections, to 75.6 for men and 80.6 for women. The national mortality rates are shown, for selected ages and for selected years of the projection, in Appendix B.

For local areas, the assumed national mortality rates have been adjusted to take account of local variations observed in the three year period preceding the projections. The local scaling factors used to adjust the national rates are shown in Appendix C.

Migration

Assumptions about future levels of migration at Scotland level 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, 2000-based, projections assume inward and outward migration in balance from 2002-03 onwards; the previous, 1998-based projections, assumed a loss of 1,000 persons a year in the longer term. Within this change Scotland is assumed to gain more migrants from overseas and lose more to the other UK constituent countries.

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, but where appropriate also take account of known events that will affect the population distribution. The assumptions for each area are shown in Appendix D. It is important to remember that the local assumptions have to be constrained to the totals used in the national projections and that for the longer term, migration forecasts are highly speculative.

Health board area (HBA) specific age/sex distributions have been assumed for the in- and out- migrant flows at that level. The distributions are based on an analysis of recent movements recorded by the National Health Service Central Register and are controlled to the age/sex distribution used for Scotland in the national projection. Where necessary, the age/sex distributions for council areas have been disaggregated from HBA figures taking account of patterns observed in 1991 Census migration data.

All statistical publications