1999 & 2000 Small Area Population Estimates, Scotland

Explanatory Notes

This paper presents two sets of small area population estimates, consistent with the Registrar General's mid-year population estimates (MYE):

1. 2000 small area estimates by gender and five-year age group for postcode sectors and wards
2. The 1999 small area estimates published in March 2001 have been updated to provide estimates by ward in addition to postcode sectors

Contents

• 1. Introduction
• 2. Background
• 3. Deriving Small Area Population Estimates
• 4. About the Datasets
• 5. Health Warnings - Uses and Limitations
• 6. Contact Details
• ANNEX A
• ANNEX B
• ANNEX C
• ANNEX D

1. Introduction

In March 2001, National Records of Scotland (NRS) delivered a set of small area population estimates for Scotland, consistent with the Registrar General's 1999 MYE. Estimates by gender and five-year age group for postcode sectors were published. Since then, NRS has digitised the 1999 ward boundaries required to map postcodes to wards, and the 1999 estimates have now been updated to include 1999 ward estimates. In addition, 2000 estimates are now available for postcode sectors and wards, by gender and five-year age group. The estimates can be downloaded from Annex D as Microsoft Excel 97 files. Alternatively, tables are available from the address in Section 6.

There are concerns about the quality of the estimates and a NRS project has been set up to address these issues (the Small Area Population Estimates (SAPE) project). In the meantime however, it is felt that their value to users outweighs the improvements in quality that could be gained by waiting for the completion of the developmental work. NRS  are therefore publishing these estimates as experimental datasets for user consultation. The estimates presented here may be used subject to the conditions outlined in Annex A.

It is recognised that many users are in a better position to evaluate the dataset, given their in-depth knowledge of their local areas using local datasets (such as the Voluntary Population Survey) and NRS requests users to provide feedback on the quality of the estimates. This feedback will help NRS to gain a better understanding of the quality of the data and will help to direct the work on improving the methodology and the underlying data sources. Feedback on the quality of this dataset should be forwarded to the address in Section 6.

2. Background

2.1 Permission
The use of the Community Health Index (CHI) as a possible data source from which to derive small area population estimates has long been recognised. Only after work to introduce a unique patient identifier in Scotland in the mid-nineties did it become realistic to begin looking at the data and determining the feasibility of its use in small area estimates. With the approval of the then Chief Area Medical Officers and the CHI Project Board, Information Services Division (ISD) provided an extract of CHI data to NRS for analysis.

As stipulated by the CHI Project Board, the extract excluded all patient names for data protection reasons, but included: address details to enable accurate geo-referencing where there were difficulties with the recorded postcode; the patient's CHI number to enable further developmental work on improving estimates of migration; and details of the patient's age and sex. The use and storage of the data is monitored by the Department's Caldicott Guardian for health data, the Deputy Chief Medical Officer, and complies with the NRS confidentiality guidelines.

In addition to the earlier agreements, a new protocol has been agreed with the fifteen Directors of Public Health, which replaces the earlier agreements and provides additional support for the publication of results derived from the CHI. The new protocol also provides further restrictions on the data to ensure patient confidentiality, such as encryption of the CHI number.

2.2 CHI Working Group
A working group was established to evaluate the CHI and make recommendations on its use. The group was chaired by NRS with members representing local authorities, health boards, Scottish Government Health Department and the Information & Statistics Division of the Common Services Agency. The Project and working groups reported to the Scottish Census Advisory Group (SCAG) and the NRS Statistics Steering Group (SSG).

The working group consulted users about their requirements from small area population estimates and determined that population estimates for postcode sectors and 1999 wards, by gender and five year age groups would satisfy the large majority of user requirements. However, additional requirements for non-standard age groupings and a more flexible geography were noted. These issues will be addressed by the SAPE project, which aims to further develop the NRS small area population estimates methodology.

3. Deriving Small Area Population Estimates

3.1 Data sources
The derivation of these small area population estimates relies mainly on the Registrar General's Mid-Year Estimates (MYE) and data from the Community Health Index (CHI). This section provides a brief outline of the MYE, along with background information about the CHI.

3.1.1 Registrar General's Mid-Year Estimates of Population
Each year the Registrar General for Scotland produces a population estimate for Scotland and its administrative areas, by age and sex. This estimate is used by a variety of users within central and local government, the health service, academia and the private sector. The estimates help to determine the allocation of resources between central government and councils and health boards; they are of fundamental importance to staff in central and local government when planning, delivering, and monitoring the provision of a wide range of services.

The starting point for the NRS estimates is the Census of Population. After adjusting for under-enumeration and differences in timing, the census is used to give figures for the base year (currently 1991) which are updated each year until the series is rebased after the following census. Each year the census estimate is rolled forward by adding births, subtracting deaths and adding net migration.

The total number of births and deaths, along with details of age, sex and location of the birth or death, is taken from the civil registration system managed by NRS. This data is considered to be virtually complete and accurate.

The estimate of migration is much more uncertain as there is no wholly reliable source from which to estimate migration. Instead migration is estimated from the best available data sources, namely: The International Passenger Survey, the National Health Service Central Register, the Electoral Roll and the School Roll. In fact, the CHI replaced the School and Electoral rolls in estimating local authority level migration for the 2000 MYE.

Using information from these data sources a 'top-down' estimate of migration for Scotland and its administrative areas is prepared. That is, a net migration total is estimated for Scotland, then net migration for each of the health boards is estimated and constrained to the Scotland total. Finally, an estimate of net migration for local authorities is derived and constrained to the health board totals.

3.1.2 Community Health Index
The main functions of the CHI are to (a) hold data facilitating payment to GPs, (b) enable a patient's medical records to be transferred with the patient as (s)he migrates from place to place, and (c) provide a common data repository for other healthcare satellite systems such as Child Health System. The CHI is also a fully integrated part of population screening and immunisation programmes.

The nearest equivalent on the CHI to the resident population definition as used in the NRS MYE is those patients who are 'currently registered' with a GP. These are people who are registered with a GP for whom the GP receives regular payment.

Table 1 highlights the differences in definitions of sub-populations in the 'currently registered' and the 'usually resident' populations.

Of these definitional differences, only the UK Armed Forces (estimated to be about 16,800 persons in 2000) might have a substantial effect. In time, NRS will investigate the need for individual adjustments for the other sub-groups. However, because of the lack of data and the relatively small populations involved, NRS assumes that these will not have a significant effect on the interpretation of the results.

The September 2000 CHI extract provided to NRS contained just over nine million records, which constitutes all records ever placed on the CHI. To obtain the currently registered population it is necessary to apply a number of criteria or filters to all of the patient records on the extract. Details of the filters used to define the currently registered population are given in Annex B.

3.2 Geography

3.2.1 Geography in the Dataset
Consultation with users indicated that small area population estimates produced for postcode sectors and 1999 wards would satisfy most user requirements. These geographies are determined by matching the patient's current postcode from the CHI against the NRS Postcode Index. The postcode index is a computer file derived from NRS digitised postcode boundaries, which provides an allocation of live and deleted small and large user postcodes within Scotland to higher areas such as electoral and administrative areas and other information including one metre national grid references. This match determines if a postcode is valid and provides area codes (Council areas, health board areas, etc.) for each valid postcode.

Postcode sectors are not contiguous with health board or council boundaries. Therefore, postcode sectors have been split to nest within health board and council boundaries. Where postcode sectors are split, a naming convention has been used to identify that it is a split postcode sector and which identifies the health board and council area the split postcode sector lies within. For example, taking the fictitious split postcode sector of KY15 7_XXYY - 'XX' is a number between 01 and 15 representing the health board, and 'YY' is a number between 01 and 32 representing the Council area. Codelists for the health boards and council areas are provided in Annex C.

3.2.2 Postcode quality
Approximately 99 per cent of the records on the CHI have a valid postcode. As the remaining one per cent of records are considered to be part of the currently registered population, they must be assigned postcodes or area markers to enable them to be used in the estimation process. This assignment uses a donor imputation method, which assumes that patients with similar characteristics will live in similar geographic locations.

For each patient record without a valid postcode, a similar record with a valid postcode is selected at random. The similarity of records is determined by comparing details of GP practice code, age, sex and health board of residence. The record with the valid postcode is the donor, and its postcode (and subsequent area codes) is assigned to the record with the invalid postcode.

3.3 How are the small area estimates calculated?
The small area estimates presented here have been constrained to the NRS MYE as these are generally considered to be the best estimates of the population. It is also felt that small area estimates need to be consistent with the NRS MYE to avoid any confusion over which population estimates should be used for certain geographies.

To meet the main geography requirement (postcode sectors and 1999 wards), population estimates are needed at the lowest building brick. That is, a population estimate, by postcode, sex and single year of age for each unit postcode. This enables different aggregations of the building bricks to produce different geographies. However, data at the building brick level will not be made available to the public because of confidentiality considerations and the likely inaccuracy of the data to such detail.

Finally, as shown in Table 1 the CHI does not include the AF population and therefore the CHI can be used only to derive estimates of the civilian population. Data from the 10 per cent sample of 1991 Census records is used to derive building brick level estimates of the non-civilian population. These two components are then added together to produce population estimates that are consistent, by sex and single year of age, with the NRS mid-year population estimates at the council, health board and national level.

4. About The Datasets

When the 1999 small area estimates were initially published in March 2001, users were provided with SuperTABLE software to enable tabulation and manipulation of the dataset. However, because of licensing restrictions, the small area estimates will no longer be delivered via the SuperTABLE software. Instead, data will be provided to users as Excel pivot tables. For users who are unfamiliar with pivot tables, brief instructions for manipulating the data are provided in Annex D.

5. Health Warnings - Uses and Limitations

The small area estimates have been largely derived from a dataset that is subject to varying degrees of list inflation in different age-sex groups and geographies throughout Scotland. The method assumes that the list inflation is equally distributed amongst postcode sectors within each council part-area. This assumption is considered reasonable, but in some areas higher concentrations of list inflation may skew the distribution of the CHI to smaller areas and age groups. Therefore, the estimates need to be interpreted with care. The information will be acceptable in terms of gaining an understanding of the proportionate size and structure of the population in postcodes sectors and wards, but will not provide definitive numbers of the population in these areas.

In addition to the mid-year estimates published for Council and Health Board areas, NRS also makes available population estimates at the former district level to users in local authorities that are an amalgamation of former districts (e.g. Fife). As these small area estimates have been constrained to council part area totals, these estimates may not be consistent with district totals. Although it is not possible to precisely amalgamate postcode sectors into district totals, some very rough district estimates should be possible. Users should be aware of the inconsistency and avoid amalgamating the postcode sectors presented here as estimates for former districts.

Although the population estimates are presented in units, this does not imply accuracy to that level. The data are presented in units for the convenience of users wishing to compile non-standard aggregations without encountering rounding problems.

6. Contact Details

Any queries about the statistics in this dataset, or feedback on the quality of the estimates as indicated in Section 1, should be directed to Statistics Customer Services using our Contact us page.

Annex A

Conditions of Use
All material contained in this dataset and information of any kind derived from the statistics or other material in the dataset is CROWN COPYRIGHT and may be reproduced only with the permission of the National Records of Scotland (NRS).

NRS is prepared to allow extracts of statistics or other material from this report to be reproduced without a licence, provided that these form the part of a larger work not primarily designed to reproduce extracts, and provided that any extract of statistics represents only a limited part of a table or tables, and provided that Crown Copyright and source are prominently acknowledged. NRS reserve its rights in all circumstances and should be consulted in any case of uncertainty. Enquiries about the reproduction of material should be directed to NRS at the address given in Section 6, and reproduction might require a licence and fees.

Annex B

Filters For Defining 'Currently Registered' Population

1. Exclude all records with a deletion indicator
   The deletion indicator refers to records that have been deleted because they have been entered onto the database in error. The records are not completely deleted but archived and are inaccessible to most users. Approximately 8,000 out of the 9 million records have such a deletion indicator.

2. Exclude all records without a GP Practice code
   Records without a GP practice code are considered not to be registered with a GP. For example, some hospitals may enter patients on the CHI for local administrative purposes but they are not registered with a GP and are therefore excluded from the 'currently registered' population.

3. Exclude all records where the date of transfer out is not blank
   Excludes those records, which are recorded as transferring off a GP list. A patient is transferred off a GP list for a number of reasons, such as migration out of the area or joining the Armed Forces.

4. Exclude all records where the date of death is not blank
   Excludes patients who died before 1 July 1999.

5. Exclude all records where the UPI status is not blank or 'C'
   Excludes all of a patient's previous records. For instance when a patient registers with a new GP in another area, a new record is created and this indicator will identify the current record as 'live' and the previous record as 'historical'.

Annex C

Health Board & Council Area Codelists

Annex D

The Tables
Concerns have been raised about a) the possible identification of individuals and b) the implied accuracy of the data if users were permitted to view certain types of table - for example, ward by postcode sector. The estimates therefore, have been split into four pivot tables for each year. These tables can be downloaded as Microsoft Excel files by clicking on the names below:

1999 council area by postcode sector, sex and 5 year age group
1999 health board by postcode sector, sex and 5 year age group
1999 council area by ward, sex and 5 year age group
1999 health board by ward, sex and 5 year age group

2000 council area by postcode sector, sex and 5 year age group
2000 health board by postcode sector, sex and 5 year age group
2000 council area by ward, sex and 5 year age group
2000 health board by ward, sex and 5 year age group

In the postcode sector tables, where a sector lies across a council or health board boundary, it is given a split indicator. For example, the sector AB12 3_0201 is in health board 02 (Grampian) and council area 01 (Aberdeen City), while the sector AB12 3_0202 is in health board 02 (Grampian) and council area 01 (Aberdeenshire). A number of these split sector areas are small, with a correspondingly small estimated population. These areas have been included in the 1999 and 2000 postcode sector estimates to allow them to nest into Council and Health Board areas, but we do not intend to imply accuracy at that level. This issue will be addressed by the SAPE project over the coming months.

Each of the tables can be displayed using either area names and/or their codes. However, users should take care when displaying wards using the ward name only, as there are several wards whose names are not unique. For example there are five Scottish wards named Milton. If the table 'Ward name by age and sex' is displayed, the total for Milton will be the sum of all five wards. This can be avoided by either displaying the ward names within their councils or health boards, or by displaying wards using the name and code.

Manipulating Data with Pivot Tables
A pivot table is an interactive table that quickly summarizes, or cross-tabulates, large amounts of data. Rows and columns can be rotated to display different summaries of the source data, the data can be filtered by displaying different pages, or the details for areas of interest can be extracted into a separate table. A pivot table contains fields, each of which summarizes multiple rows of information from the source data. By dragging a field button to another part of the pivot table, you can view your data in different ways. For example, you can view the Health Boards either down the rows or across the columns.

Dragging Fields
The grey cells containing the pivot table field names can be dragged to a new position to change the way the data is displayed. For example, the fields 'age group' and 'sex' can be swapped to give sex by age rather than age by sex.

Adding or Deleting Totals
To add or delete subtotals to/from a field, double click on the cell containing the field name to display the pivot table field options below. From here the orientation of fields can be changed, totals added or deleted, sort orders determined and items hidden.

Adding or Deleting Fields
Fields may be deleted directly on the table by right clicking a field name and selecting Delete. To add fields to a pivot table, right click on a cell of the table and select Wizard ... This brings up the pivot table wizard (below). The data to appear in the pivot table can be modified by dragging the field buttons on the right to the row, column and page sections in the diagram on the left.