Migration bias in ecologic studies

Differential migration may provoke bias in an epidemiological assessment of the public health risks from exposure to environmental agents, particularl y in ecologic studies of health outcomes with a long latency or induction p eriod. The potential impact of migration bias on epidemiological research i s complex, and it depends not only on the direction of the factor-related m igration, but also on its extent. This study shows that even a small amount of differential migration can bias the assessment of the exposure-outcome relationship. Migration bias may result from a number of circumstances that are related to the way in which 'populations' are defined and ascertained. It is important to understand and minimise this type of bias in epidemiolo gical research.