Internal Migration Intensity Index
Also known as: Crude Migration Intensity, Aggregate Crude Migration Intensity, CMI / ACMI, Migration Intensity Measure
The internal migration intensity index measures how much migration occurs within a country — the overall propensity of people to change their place of usual residence — independently of where they move or what the net redistribution is. Its simplest form, the Crude Migration Intensity (CMI), is just the number of internal migrants over an interval divided by the population at risk, expressed per hundred or per thousand. Martin Bell and colleagues, in their 2002 Journal of the Royal Statistical Society paper and the later IMAGE programme, showed that this apparently simple measure is treacherous to compare across countries because it depends heavily on how the territory is carved into zones: the more, smaller regions you define, the more boundary-crossing 'migrants' you count. They therefore developed the Aggregate Crude Migration Intensity (ACMI), a scale-standardized intensity that corrects for this modifiable areal unit problem, and embedded it in a four-part framework — intensity, impact, pattern, and distance — that separates the overall amount of migration from its net effect, its spatial structure, and its typical range. Grounded in the measurement conventions of the United Nations migration manuals, the index gives demographers a defensible, internationally comparable answer to the deceptively hard question 'how migratory is this population?'
Key highlights
- Provides a single, interpretable measure of overall propensity to migrate internally, independent of direction or net effect.
- Explicitly confronts and corrects the modifiable areal unit problem, making cross-national comparison legitimate via the aggregate (ACMI) form.
- Sits within a coherent four-part framework (intensity, impact, pattern, distance) that prevents conflating distinct features of a migration system.
- Built on standardized United Nations migrant definitions, so it travels well across data sources when those conventions are followed.
Intuition
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How it works
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When to use it
Use the internal migration intensity index when you want a single, defensible summary of how migratory a population is and, especially, when you need to compare overall mobility across countries, regions, or time periods. It is the appropriate tool when the data are zonal counts of migrants and at-risk population, and when differences in statistical geography or interval length would otherwise make raw comparisons misleading. It is also valuable as the 'intensity' pillar within a fuller four-dimensional characterization of a migration system. The index is less useful when you care chiefly about the direction or net redistribution of migration (use effectiveness and net-rate measures instead), about specific origin-destination patterns, or about international rather than internal migration. It also presupposes consistent migrant definitions and reasonably good population denominators, which not all data sources supply.
Strengths & limitations
- Provides a single, interpretable measure of overall propensity to migrate internally, independent of direction or net effect.
- Explicitly confronts and corrects the modifiable areal unit problem, making cross-national comparison legitimate via the aggregate (ACMI) form.
- Sits within a coherent four-part framework (intensity, impact, pattern, distance) that prevents conflating distinct features of a migration system.
- Built on standardized United Nations migrant definitions, so it travels well across data sources when those conventions are followed.
- Raw crude intensity is strongly scale-dependent and meaningless across systems without the areal-unit adjustment.
- Requires migrant counts at multiple spatial scales to estimate the aggregate intensity, which many sources do not provide.
- Sensitive to the interval length and to whether a transition or event definition of migration is used, complicating harmonization.
- Captures only the amount of migration, telling you nothing by itself about net redistribution, spatial pattern, or distance.
Common pitfalls
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Applications
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Frequently asked
Why can't I just compare crude migration rates between countries?
Because the crude rate depends on how each country divides its territory. A country split into many small zones will record more boundary-crossing moves as 'migration' than the same country split into a few large zones, even if people move identically. This modifiable areal unit problem means raw crude intensities reflect statistical geography as much as real behaviour. The aggregate crude migration intensity corrects for this by measuring intensity across scales and standardizing, so comparisons reflect genuine differences in mobility rather than artefacts of zoning.
What is the difference between intensity and impact?
Intensity measures how much migration happens — the share of people who move — regardless of direction. Impact measures how much that migration actually redistributes the population, captured by effectiveness and net-rate measures. The two can diverge sharply: a population can have very high intensity (lots of churn) but low impact if inflows and outflows roughly cancel, or modest intensity with high impact if flows are strongly directional. Keeping them distinct, as Bell and colleagues insist, is central to characterizing a migration system correctly.
Does the time interval matter for the index?
Yes, considerably. A five-year interval captures more moves per person than a one-year interval, and it also misses repeat and return moves within the window, so intensities from different intervals are not directly comparable. The United Nations conventions and the IMAGE programme stress harmonizing the interval, the migrant definition (transition versus event), and the at-risk population before comparing. Where intervals differ, analysts must convert or model them onto a common basis rather than compare the raw numbers.
Sources
- 1.Bell, M., Blake, M., Boyle, P., Duke-Williams, O., Rees, P., Stillwell, J., & Hugo, G. (2002). Cross-national comparison of internal migration: issues and measures. Journal of the Royal Statistical Society: Series A, 165(3), 435-464.
- 2.United Nations (1983). Manual on Methods of Estimating Internal Migration (Manual VI). Population Studies No. 47. New York: United Nations.
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Cite this page
ScholarGate. (2026, June 23). Internal Migration Intensity Index. ScholarGate. https://scholargate.app/migration-studies/internal-migration-intensity-index