The standard approach to subnational projection
Projections for subnational populations are routinely carried out by both national and international organisations. The outputs are the populations of regions classified by age and sex for years in the future. The methodology for handling such projections has been under development for some three decades. At its heart, it contains a multiregional model, the essentials of which were proposed by Rogers in 1968 and developed by him and others in the 1970s (Rogers, 1975, 1995; Rees & Wilson 1977; Stone, 1976).
Difficulties faced by the standard approach
However, there have been difficulties in implementing the multiregional model when the number of regions is high. These difficulties include:
So what features need to be added to the standard multiregional model so that its key advantage (that it captures the mutual interaction between regional populations) can be exploited?
The model needs to be embodied in a system of information processing with the following features:
The UKPOP projection model for the new UAs
The paper will describe a system for projecting the populations of the new unitary authorities of the United Kingdom. To carry out that task a raw database is assembled at a variety of spatial scales (district and ward) that contribute to the new areas. Data from the last census in 1991 or from 1990 and 1991 calendar years is used to construct estimates of the detailed input variables, while data for districts for 1992-95 is used to update the input variables.
Raw data base for the recent past
Routines for geographical conversion of the raw data
Efficient routines are needed to effect the geographical conversions outlined above. These involve transparent look-up tables.
Routines for estimation of model variables from raw inputs
These routines are mainly needed for adding age-sex disaggregation at the finest spatial scale where such data are absent or unreliable. The technique of reverse standardisation is used.
Routines for derivation of key diagnostic indicators
Efficient routines are needed for generating fertility indices (total fertility rate, general fertility rate, standardised fertility indices), mortality indices (mean and median life expectancies, standardised mortality indices), and migration indices (gross migraproduction rates, standardised migration indices), because they can potentially be used with 204 UAs (32 in Scotland, 22 in Wales, 149 in England (provisional), with 1 assumed in Northern Ireland).
Statistical analysis of the key effects in the estimated database
The key question which statistical analysis helps answer is as follows. What reduced form model can be used to estimate the full array of migration probabilities used in a multiregional projection model? The dimensions of the model can be represented by origins (O), destinations (D), ages (A), sexes (S) and time (T)?
A saturated model would use all array probabilities:
Redesign of the multiregional model that builds on the statistical analysis
The model uses an extension of the hierarchical design used by Rees (1996) for NUTS 1 regions in the European Union. In that ECPOP model two levels were recognised in the hierarchy: member states of the European Union and regions within member states. In the UKPOP model the two levels are home countries (England, Wales, Scotland and Northern Ireland) and UAs within home countries. This design recognises the country division of national projections (carried out by the Government Actuary and the Office for National Statistics) and the fact that the subnational projections are carried out by separate bodies (ONS for England, Welsh Office for Wales, General Register Office for Scotland and no projections for Northern Ireland).
Development of scenarios for input variables
The UKPOP model provides flexible means of designing scenarios using either the detailed input variables or summary indicators. Experience has shown that full flexibility is needed for projections that need to be tweaked to reflect minor local factors but that generality is needed for carrying broad brush scenario exploration.
Routines for the reporting and display of model outputs
Because the results of subnational projections are so voluminous, methods are needed for displaying the information in an efficient way under user control. The UKPOP model uses user selected parameters to control output, but the task of interfacing to visualisation software is left to the future.
Routines for constraining outputs, either top-down or bottom up
A final requirement of a projection model for many regions is that the outputs be adjusted in a consistent way to external constraints. In a hierarchical model, the national population may be projected independently of the subnational. It is then possible to adjust the subnational results to the national. In the UKPOP model, an option is included for constraint of model outputs to external constraints (such as the GAD/ONS projections).
The flow of information into, round and out of population projection software must be planned with care. The UKPOP model, implemented for a new set of geographic areas, illustrates the kind of strategies needed to achieve a reasonable compromise between the unreasonable information demands of a conventional multiregional model when over 200 regions are being used and complete parsimony which neglects important migration behaviours. The UKPOP model provides, for the first time, an integrated projection of subnational populations for the whole of the UK using the same model and assumptions.
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