Household Projections for Wales (2006-based): Frequently Asked Questions
General
What are projections? Is this another word for forecasts?
Household projections provide estimates of the future number of households, and are based on assumptions about future population growth and household composition and size. The assumptions are based on past trends and the projections only indicate what may happen should recent trends continue. They are not forecasts. See the Guidance Leaflet for further information.
How certain can we be of the results of the projections?
These are trend based projections which indicate what may happen should recent trends continue.
How do we know if these projections are right?
Household projections are estimates of the future household numbers, but they are not predictions. They are based on population projections which are themselves based on assumptions about fertility, mortality and migration. Changing these underlying assumptions would lead to different results. To illustrate this, household projections have been produced based on the available variant population projections. Data from the variant household projections are available on request.
When will the household projections be updated?
The population projections on which the household projections are based are updated every two years and it is anticipated that they will be updated on a similar basis. Consequently, we expect the next set of household projections to be 2008-based and to be published in late 2010.
Are these projections of the number of houses which need to be built?
No. These are projections of the number of households, not the number of new houses required (housing need). There may be links between the two, but the definitions are different. A household is defined as a group of one or more people living at the same address and sharing a living room or one or more meals a day. Although in most cases a house contains a single household, there are instances where more than one household lives in the same house (e.g. where it has been divided into bedsits to become a house in multiple occupation (HIMO)).
Do these household projections take into account the effects of the current recession?
No. These household projections are largely based on continuing past trends in household membership rates and in the underlying population projections. Due to the data used these household projections are in effect based on long term trends in household formation. They do not take into account any effect the recent economic downturn may have on household membership rates or underlying demographic trends. It should be noted though that the effects of previous recessions will have been picked up through the use of data from the last two censuses to project future household membership rates.
Do these projections take into account proposed housing developments in each area?
No. These are trend-based projections. Projections done in this way do not make allowances for the effect on future household growth of changes to local or central government policies or general economic conditions.
Do these projections take account of student numbers and changes to the size of intakes to higher education institutions?
The projections are based on the usually resident population, with students being counted at their term-time address. As the projections are based on recent trends, no allowance is made for planned changes to HE institution intakes in the future. Essentially, the projections assume the existing or recent level of HE students as they are largely taken account of in the migration figures.
Do these household projections take into account changes in migration?
Migration trends have been taken account of in the population projections on which the 2006-based household projections are based. The international migration assumptions for the local authority population projections were based on trends in recent years.
A substantial amount of work is being taken forward within the ONS to improve international migration estimates. Any identified improvements to migration estimates will be included in future projections. Work is also planned on the feasibility of treating students as a special population in the next set of projections. This work will involve considering new HE administrative data.
What about National Park projections?
Although some work has been done to produce population projections for the 4 national park areas in Wales this is purely experimental and as yet no plans have been made to produce household projections on the same basis.
Using the projections
Will a Local Authority be expected to use these household projections in their Local Development Plan (LDP)?
Not necessarily. The projections form part of the evidence base needed to produce an LDP. They should form the starting point; however, because the projections are trend-based, local planning authorities may wish to produce their own policy-based projections, in which case they should justify the approach they have taken. They should also use any other information available to inform the evidence base such as local housing market assessments. The Housegroup Wales software package has been provided to enable planning authorities to produce localised forecasts or projections.
Are Local Authorities expected to adhere to these household projections in their planning of services?
No. We have produced household projections using a consistent basis and assumptions for all local authorities. These are largely based on continuing past trends in household membership rates and those from the underlying population projections. The projections are a starting point, but local authorities may have information which suggests alternative future trends should be used.
How have the projections been calculated?
How are projected membership rates calculated?
Projected membership rates have been calculated using census data for 1991 and 2001 only. Data from the Annual Population Survey was considered for this purpose but found to be unsuitable. As a result household membership rates have been projected using a two point exponential model.
What are the key differences between the methodology used in Wales and England?
Household projections for both England and Wales used the same methodology up until the 2003-based household projections. For the 2006-based household projections the key differences are:
- Household projections for England continue to be based on headship rates whilst for Wales they are now based on membership rates.
- Household projections for England continue to use household types based on marital status whereas for Wales household types are now based on household size and the presence of children.
- For England projected household numbers by local authority add up to the projected numbers at a national level, whereas for Wales this is not the case. This is because separate local and national household projections have been produced using the available local authority population projection data from the Welsh Assembly Government and national population projection data from the Office for National Statistics.
What is the difference between headship and membership rates?
Both membership and headship rates can be calculated from historical population data and applied to population projection data to produce household projections.
Headship rates are based on the probability that a person of a particular age and gender will be the head of a particular type of household.
Membership rates are based on the probability that a person of a particular age and gender will be a member of a particular type of household.
So for each combination of age and gender (e.g. males aged 0-4, females aged 19-24, etc.) rates can be calculated and projected forward for each household type, even if they are zero in some cases. For both the headship and membership methods the projected rates can be applied to the projected private household population to produce the household projection.
For Wales why have membership rates been used instead of headship rates?
The use of membership rates was chosen because it avoids the effect of changes to the definition of the head of a household which can occur between censuses. The use of membership rates can also provide more information on the individual members in each household type (e.g. household population by age and gender as well as household type).
The projected household numbers are published to the nearest 1. Can they accurately be projected in this way and how should they be presented?
The projected household numbers are shown to the nearest 1 because they have been calculated by individual household type before being aggregated. They are published on the same basis to give users greater access to the data. Nevertheless when using the figures in reports etc. it is advisable to avoid overreliance on the exact figures by rounding them to the nearest hundred.
Why have separate household projections been produced for local authorities in Wales and for Wales as a whole?
The 2006-based household projections for Wales have been produced using the population projection data available at a national and local authority level. By using both sets of data it has been possible to produce an all-Wales household projection which is as consistent as possible with that for England as well as results by local authority.
This of course means that by summing the local authority results it is possible to obtain projected total household numbers for Wales which are different to the published totals. Further guidance on when to use the national and local authority household projection figures is provided in the National and Local Projections section of the Summary Report.
Quality Assurance
What measures have been taken to quality assure the household projection outputs?
Colleagues from the Office for National Statistics were consulted on the development of the methodology. Then, once the provisional results had been produced an event was arranged to quality assure them. This was held under National Statistics restrictions and enabled local authorities in Wales to provide a local knowledge perspective on the credibility of the results and to raise actions/questions for further investigation. The methodology was also peer reviewed by external housing research experts.
HOUSEGROUP WALES
Why has a new version of the HOUSEGROUP package been created?
The original version of HOUSEGROUP was based on headship rates and so could not be used with the membership rate methodology used in the 2006-based household projections for Wales. A new version, named HOUSEGROUP WALES, was commissioned to accommodate the new methodology, to make the projection results more accessible and to enable users to produce their own forecasts using alternative assumptions or data. This can be provided free of charge on request.
How can I get a copy of HOUSEGROUP WALES?
A form is available for potential users to download and email in order to obtain the software. The purposes of this are to monitor the user base and provide future updates, and the software is available free of charge.