6. Sources and Methods

6.1 Summary

6.1.1 These are the first set of household projections to be produced since results from the 2011 Census became available. We have reviewed the method used to produce the projections to decide how best to incorporate the new census data. This review has led to significant changes in the way that information on the types of households that people live in is projected forward. These changes mean longer-term trends have been incorporated, and the method now has the flexibility to make adjustments to the census-based trends in household formation based on more up-to-date survey information. This becomes increasingly important as the base year of the projection moves further away from the most recent census. A brief description of the method is given below, and more detailed information is provided in the following sections. A flow diagram illustrating the new method is shown in Figure 22.

6.1.2 The household projections are based on population projections produced by the National Records of Scotland (NRS). The number of people living in private households is estimated by taking the population projections for each year and subtracting the number of people living in communal establishments, such as student halls of residence, care homes or prisons. To estimate the number of households of each type, information on household formation is projected forward from the 1991, 2001 and 2011 Censuses, for each household type, age group and Council area. This information is then applied to the private household population to produce the basic household projections.

6.1.3 The overall projections for Scotland are believed to be more accurate than those for individual Council areas; therefore, the Council area projections are constrained to the Scottish total. Each year NRS produces estimates of the total number of households in each Council area, based on Council Tax data. These estimates are based on more recent data than the household projections therefore the household projections for 2012 and 2013 are adjusted to match the household estimates (on the NRS website), and for 2014 onwards the projections are adjusted by the same proportions as 2013.

Figure 22: Flow diagram of the method used to produce household projections

Image showing flow diagram of the method used to produce household projections

6.2 Household types and age groups used in the household projections

6.2.1 Household projections are produced for each Council area, broken down into seven household types, based on the number of adults and children living in the household, and sixteen age groups, as follows:

Table A: Household types and age groups used in the household projections
Household typesAge group
1 person households:

1 adult: male
1 adult: female

2 person households:

2 adults
1 adult, 1 child

3+ person households:

1 adult, 2+ children
2+ adults, 1+ children
3+ adults
16-19
20-24
25-29
30-34
35-39
40-44
45-49
50-54
55-59
60-64
65-69
70-74
75-79
80-84
85-89
90+

6.3 Data sources

6.3.1 Five sets of data are used as inputs to the household projections:

Population projections

6.3.2 The first input into the household projections is the 2012-based population projections for Scotland. These are produced for Scotland by Council area, sex and single year of age, using assumptions about births, deaths and migration. More information about the population projections, and the assumptions used to produce them, can be found in '2012-based Population Projections for Scottish Areas' on the NRS website. The relevant population for household formation is taken to be the adult population, aged 16 or over. The low and high migration variant population projections used in the variant household projections are taken from the same publication.

Communal establishment estimates

6.3.3 Estimates of the proportion of the population living in communal establishments, such as care homes or prisons, are used to derive the number of people living in private households from the total population given in the population projections. These proportions are calculated, by age group and sex, from the mid-2012 population estimates published by NRS and numbers of residents in communal establishments collated by NRS.

6.3.4 Numbers of residents in communal establishments are collected from a range of data sources, depending on the establishment type. The data is chosen to represent, as closely as possible, the census definition of residence, that is those individuals 'staying, or expecting to stay, in a residential establishment for six months or more'. Individuals resident for shorter stays would be considered visitors and are not included, as they should be accounted for in their usual place of residence.

6.3.5 Data is collected from a range of administrative data sources and surveys and refers to 2012, where possible. For some establishment types, however, no such source is available, and in these cases 2011 Census data is used. Table B details the sources of communal establishment data and the year to which the data refers. In many cases, more than one data source was combined and estimation was required to obtain a full age/gender breakdown for all establishments.

6.3.6 The communal establishment rates used for the 2012-based household projections can be found on the NRS website, in the 'Source Data Tables'.

Table B: Communal establishment data sources
Establishment typeData sourceYear of data
Footnotes:
  1. For this establishment type data from tw o or more sources are combined to estimate the age/gender breakdow n of residents in each Council area, as no comprehensive source w as available.
  2. ISD - Information Services Division of NHS Scotland
  3. DASA - Defence Analytical Services and Advice
  4. HESA - Higher Education Statistics Agency
  5. Further information on the communal establishment data collection and the uses of this data can be found in the methodology section of the 2012-based household projections for Scotland.
Adult care1Care Inspectorate (List of registered establishments)2012
ISD2 Care Home Census2012
Children's care1Care Inspectorate (List of registered establishments)2012
SG Looked After Children Statistics2012
Defence1NRS Armed Forces Data Collection2012
DASA3 Regular Forces by Age and Sex2012
2011 Census2011
HospitalsISD2 SMR01 (General/Acute)2012
ISD2 SMR04 (Mental Health)2010
ISD2 Continuing Care Census (Other NHS)2012
Hostels2011 Census2011
Hotels and boarding houses2011 Census2011
PrisonsSG Prison Statistics2012
Residential SchoolsSG Education Analytical Services2012
SG Directorate for Learning2012
Student Halls of Residence - Further Education2011 Census2011
Student Halls of Residence - Higher Education1NRS Communal establishment address list2011
HESA42011/12
Other2011 Census2011

Headship rates

6.3.7 The household formation information used in the household projections is in the form of headship rates, derived from the 1991, 2001 and 2011 Censuses. These rates are used to convert the private household population into household numbers.

6.3.8 In the census, one member of each household is designated the ‘head of household’ (the first adult recorded on the household form). The headship rate describes, for each age group, the proportion of the population that is designated the head of household of each household type. The proportion of the population, for each age group, who are not a head of any type of household (non-heads) is also available from the census. The proportions for each household type, including non-heads, sum to one, within each age group and Council area. The number of people who head particular household types is the same as the number of households of this type.

Household type distribution

6.3.9 For the first time, survey data has been included in the projection method and plays an important role in the projection of headship rates. The data comes from the 2012 Scottish Household Survey (SHS) and consists of an estimate of the proportion of households of each type. This data is then adjusted based on differences between 2011 SHS data and the 2011 Census to take into account higher levels of non-response from particular household types in the SHS.

6.3.10 The distribution of households across the seven types of household used in the projections for 2012 can be found on the NRS website, in the 'Source Data Tables'.

Household estimates

6.3.11 Household estimates are published annually by NRS and the household projections use the estimates from the latest two years (2012 and 2013). The estimates are based on Council Tax data, and provide the total number of households for each Council area in Scotland. The latest household estimates can be found in the 'Estimates of Households and Dwellings in Scotland, 2013' publication on the NRS website.

6.4 Methodology

6.4.1 There are several stages to the calculation of the household projections. The first stage calculates the private household population. This is done by first estimating the number of communal establishment residents in each year of the projection, by applying the communal establishment rates described above to the population projections. The communal establishment residents are then subtracted from the population projections to give the private household population for each projection year. The projected private household population for 2012 to 2037, for Scotland and each Council area, can be found on the NRS website, in the 'Source Data Tables'. Variant private household populations are also available in the same dataset, based on the high and low migration variant population projections.

6.4.2 The second stage in calculating the household projections is to estimate the number of households from the private household population, using the headship rates described above. Two sets of projected headship rates are produced using a modified two-point exponential model, one using headship rates from the 1991 and 2001 Censuses, and the other using headship rates from the 2001 and 2011 Censuses. The formula for the modified two-point exponential model is as follows:

Image showing the formula for the modified two-point exponential model

6.4.3 The two sets of projected headship rates are then combined into a final set of headship rates using weights and the following formula:

Image showing formula for calculating final headship rate in year

6.4.4 The weights are chosen so that the projection for 2012 has a distribution of households across the seven household types that is as close as possible to the distribution found in the adjusted 2012 Scottish Household Survey data. This allows us to incorporate more recent information on the types of households that people are living in, rather than purely relying on census data. For the 2012-based projections the weights used were 75 per cent towards the headship rates using 2001 and 2011 Census data and 25 per cent towards projected headship rates using 1991 and 2001 Census data.

6.4.5 The projected headship (and non-headship) rates are constrained so that:

6.4.6 The projected headship and non-headship rates from the 2012-based household projections can be found on the NRS website in the 'Source Data Tables'.

6.4.7 The household projections are calculated by applying the projected headship rates to the private household population projections to give an estimate of the number of heads of household in each of the projection years for each household type, age group of the head of household and area.

6.4.8 The final stages of the household projection methodology apply a series of adjustments to the household projections. The first adjustment is to control the Council area projections to the all-Scotland projection, as the latter is thought to be more accurate. This ensures that the Council area projections sum to the total for Scotland.

6.4.9 The next adjustment is for the total number of households within each Council area in 2012 and 2013 to be controlled so that it is equal to the household estimates for these years. The household projections for 2014 onwards are then adjusted by the same proportions as in 2013, to preserve the trend in the household projections. This adjustment allows us to incorporate more recent information on household numbers than the 2011 Census.

6.4.10 The final adjustment ensures that the minimum number of adults required to fill the projected households is not greater than the projected adult private household population (e.g. a minimum of three adults would live in a ‘three or more adult’ household). The same check is carried out for children. Where an adjustment is required, the number of households is kept constant, but the balance of household types is adjusted, to reduce the number of large households and increase the number of smaller households. For the 2012-based household projections, minimum adult adjustments were required for Eilean Siar in the principal projection and the low migration variant. In the principal projection, adjustments were made for 2032 to 2037, while adjustments were made from 2030 to 2037 in the low migration variant. No minimum adult adjustments were required in the high migration variant, and no minimum child adjustments were required for any Council areas in any of the projections.