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Occasional Paper

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.

3.11 Registrar General's Mid-Year Estimate of Population

The starting point for the GROS 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 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 GROS. 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.

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 nearest equivalent on the CHI to the resident population definition as used in the GROS MYE is those patients that are 'currently registered' with a GP. The currently registered population can be defined as those patients currently registered with a GP for which the GP receives regular payment.

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

Table 1 Comparison of population sub-groups from the CHI 'currently registered' population and the mid-year estimates.

Of these definitional differences, only the UK Armed Forces (estimated to be about 16,500 persons in 1999) sub-group might have a substantial effect. In time, GROS 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, GROS assumes that these will not have a significant effect on the interpretation of the results.

The September 1999 CHI extract provided to GROS contained nearly 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 CHI (about nine million). The filters used to define the currently registered population were agreed by the working group and defined through consultation with users of the CHI and SEMA, the company managing the development and maintenance of the CHI. Details, of the five filters are given in Annex C.