Dual-Systems Estimation of Irregular Migration

Dual-systems estimation, the two-list special case of capture-recapture, estimates how many irregular migrants exist by exploiting the overlap between administrative records that each capture only part of the population. The logic is borrowed from wildlife ecology: tag animals on one trapping occasion, see how many tagged animals reappear on a second, and the rate of overlap reveals how many were never caught at all. Applied to migration, the 'traps' become administrative lists — police apprehension records, hospital registers, deportation files — and the people who appear on no list are the quantity to be estimated. Van der Heijden, Cruyff, and colleagues showed in their 2003 Statistica Neerlandica paper that even a single police register can support estimation through the truncated-Poisson model, by using how often individuals are recorded to infer how many were never recorded. Böhning, van der Heijden, and Bunge's 2018 monograph consolidated the modern toolkit for the social and medical sciences, with explicit treatment of the two assumptions that make or break the method: that lists are not too strongly dependent and that the population is not too heterogeneous in its chance of being captured. The output is a defensible estimate of an irregular-migrant total that, by definition, no register sees in full.