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APPENDIX 1 INTERNATIONAL STUDIES OF INTERNAL MIGRATION ESTIMATION - METHODS AND SOURCES United States of America The US Census Bureau estimates internal migration for states and also for counties (US Census Bureau 2009), the primary legal divisions of most states. To measure internal migration in the United States, the US Census Bureau uses data from Federal income tax returns and Medicare and makes separate estimates for the population aged under 65, and 65 and over. These estimates are combined to arrive at migration estimates for the total population. To estimate internal migration for the population aged under 65, two years of tax returns are matched and the addresses are compared to identify the number of movers from one county to another between tax filings. For persons who did not fill out a tax return, migration rates by county are assumed to be the same as those who filed a return. To measure inter-county migration for the age 65 and over population, Medicare enrolment data are used. It is assumed that the year-to-year change in enrolment represents the total change in the age 65 and over population in each county. Independent estimates of the deaths and international migration of the age 65 and over population are benchmarked to the Medicare data, with the population change not attributable to either deaths or international migration assumed to be due to net domestic migration. A Medicare-based net migration rate is calculated for each county by dividing the net internal migration estimate by the total number of Medicare enrollees at the beginning of the time period - this accounts for those who do not receive Medicare. Most administrative record data sources lag the current estimate year (by as much as two years), therefore, they project the data for the current year based on past years’ data. As updated data become available, the US Census Bureau revises the projected input data so that each year’s estimates are always based on the most recent data available. These estimates of internal migration are used to calculate annual population estimates for each US state and county, where population change is estimated by adding natural increase, net international migration and net change in group quarters population with the internal migration estimates. Canada An annual series of internal migration estimates is produced by Statistics Canada (Statistics Canada, 2008). These estimates are prepared for Census Divisions (available since 1976-77) and Census Metropolitan Areas (since 1992-93). These migration estimates are derived from a comparison of addresses from individual income tax returns for two consecutive years. The development of the data then involves four main steps:
The postal code on each tax record is matched to a geography basefile to pick up additional geographic information such as Census Division and Census Metropolitan Area. However, the use of postal codes can result in incongruity between postal code service areas and Census Divisions. The problem may arise in rural areas where a postal code service area is large and crosses one or more Census Division boundaries. Since the postal code is assigned to only one Census Division, a certain amount of misallocation can occur. There may be positive and negative misallocation that cancels out in some Census Divisions, but geographic misallocation may result in net under or over-coverage. Provincial totals are not affected by this problem. The system estimates taxfilers' dependants by linking all filing family members together and estimating non-filing members from information on the taxfilers' returns, based on information as for dependants. After accounting for the time taken to submit tax forms and processing, estimates are generally available 15 to 18 months after the income tax deadline. In addition to the estimates prepared using annual tax data, estimates of interprovincial migration are also prepared based on monthly Child Tax Benefit records. United Kingdom The Office of National Statistics (ONS) produces estimates of internal migration within the UK at the country level and at lower levels, such as Government Office Regions and Local Authorities (ONS, 2011). These estimates are derived from two administrative data sources, namely the National Health Service Central Register (NHSCR) and the Patient Register Data System (PRDS). These sources have different properties in terms of coverage and content. To estimate movements between local authorities, the NHSCR figures are combined with the more detailed PRDS. Comparing records in one year with those of the previous year by linking based on NHS number enables identification of people who change their postcode. Every NHS authority holds a record of NHS patients registered with GPs in their area comprising NHS number, sex, date of birth, date of acceptance and patient’s address. ONS receive an annual mid-year download from each authority’s register which, when combined, forms a total patient register for England and Wales. Comparing registers from consecutive years, linked by NHS number, ONS identifies all individuals whose postcode differs between the two years. These counts are then constrained to NHSCR Health Authority (HAs) level data to produce internal migration estimates. Data from the NHSCR system are limited to movements between the former health authorities. These former health authorities no longer exist as administrative entities but continue to be used due to technical constraints of the NHSCR processing system. PRDS data are also inadequate as a stand alone source as a number of within year moves are not captured. Limitations of estimates derived from a combination of NHSCR and PRDS include: variation in the delay between a person moving and registering with a new doctor; some moves may not result in a GP re-registration and therefore will not be recorded, and individuals may move and re-register more than once in a single year but remain within the same health authority Despite some limitations, research has shown that there is no other source that has as good coverage and quality as the NHS administrative data (NHSCR and PRDS) for estimating internal migration within the UK. These estimates, along with births, deaths and international migration, form the key components of the annual population estimates produced by the ONS. Document Selection These documents will be presented in a new window.
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