Internal Migration Intensity Index

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?'