Migration Effectiveness Index

Intensity measures how much migration occurs — the volume of interregional moves relative to the population at risk — regardless of direction. Effectiveness measures how directed that migration is — what share of the gross turnover ends up as net redistribution. A country can have high intensity but low effectiveness if its large flows largely cancel out, or low intensity but high effectiveness if its few moves are strongly one-way. The aggregate net migration rate combines the two, equal to one half of intensity times effectiveness, so that overall redistribution can be attributed to volume, direction, or both.

Because effectiveness is precisely the ratio of net change to gross movement, you cannot compute it from net balances alone. Two systems with identical net migration can have completely different effectiveness depending on how much gross churning produced that net result. The numerator uses the absolute net balances, but the denominator requires the total in- and out-migration — the gross flows — that the index compares the net result against. Without observed gross flows, only net migration rates can be computed, and the efficiency of redistribution remains unknown.

Quite sensitive, because they depend on which moves count as interregional. Splitting a country into many small regions captures shorter moves and generally raises measured intensity, while coarse regions count only long-distance moves; effectiveness can shift too as the balance of flows changes with scale. This modifiable areal unit problem means the indices are only comparable across systems built on similar numbers and sizes of regions. Comparative projects therefore go to great lengths to harmonize spatial frameworks and migration intervals before contrasting countries.