Despite the acknowledged significance of migration as the pre-eminent component
of population change, understanding of the way population mobility varies between
countries is, as yet, poorly developed. One symptom is that measures of migration
are conspicuous by their absence from tables of international statistical demographic
indicators. While fertility, mortality, and even international migration now
commonly appear in such lists, internal migration is invariably absent. The reasons
for this omission are well established and include the multidimensional nature of
the mobility process, differences in the way migration is measured, and problems
of spatial and temporal comparability, all of which prejudice rigorous comparative
analysis.
In responding to this deficit, analysts have focused on two main objectives: identifying
the types of migration data collected in countries around the world, and
establishing common metrics that can be used to compare mobility behaviour. For
the former, the pioneering work is due to Rees and Kupiszewski (1996, 1999a) who
took inventory of the data available in Europe. This work was later extended by
Bell (2003, 2005) to cover the 191 member states of the United Nations. For the
latter, the key proposals emanate from a joint Anglo-Australian project which identified
15 discrete indicators, in four main groups, that might be used to measure
various facets of internal migration (Bell et al., 2002; Rees, Bell, Duke-Williams,
& Blake, 2000). In parallel with these developments, attention has also been given
to some of the specific problems of migration analysis, such as changes in statistical
boundaries, which seriously prejudice temporal comparison of migration flows
(see, for example, Blake et al., 2000; Boyle & Feng, 2002; Stillwell, Bell, Blake,
Duke-Williams, & Rees, 2000).