Over-coverage in population registers leads to bias in demographic estimates.

Estimating the number of individuals living in a country is an essential task for demographers. This study assesses the potential bias in estimating the size of different migrant populations due to over-coverage in population registers. Over-coverage-individuals registered but not living in a country-is an increasingly pressing phenomenon; however, there is no common understanding of how to deal with over-coverage in demographic research. This study examines different approaches to and improvements in over-coverage estimation using Swedish total population register data. We assess over-coverage levels across migrant groups, test how estimates of age-specific death and fertility rates are affected when adjusting for over-coverage, and examine whether over-coverage can explain part of the healthy migrant paradox. Our results confirm the existence of over-coverage and we find substantial changes in mortality and fertility rates, when adjusted, for people of migrating age. Accounting for over-coverage is particularly important for correctly estimating migrant fertility.