The Effect of Migration on Comparison of Disease Rates in Geographic Studies in the United States

Abstract

Cancer rates are often compared between counties or other geographic units as a method of testing for risk from environmental exposures. Migration between geographic areas greatly reduces the sensitivity of this method. Under simplifying assumptions the quantitative effect of migration on risk estimates is shown using migration and cancer incidence data for the United States. For example, 40--50% of the relative excess risk, defined as the relative risk minus one, is not reflected in the estimated risk for most cancers, when rates are compared between exposed and unexposed counties and migration has taken place during a 30-year latent period. More extreme losses of sensitivity also occur. Under the simplifying assumptions, the quantitative effect of migration on risk estimates is shown as a function of cancer site, latent period, and the type of geographic units for which rates are calculated--states, counties, or places. Also discussed are some implications of these findings for geographically-based studies and additional data needs for assessing the effect of migration.

The effects of migration on the detection of geographic differences in disease risk

“…To minimize the effects of migration on the assessment of underlying relative risk that characterizes regions, Polissar (1980) recommends (a) study regions that are relatively large, and (b) the study of diseases characterized by relatively short latency periods. The desire for study in large regions needs to be balanced of course by the desire to better understand geographic variability within small study areas.…”

“…A model based upon Polissar's approach, with an application to cancer data from New York State Polissar (1980) examined the effects of migration on standardized incidence rates by first defining the standardized ratio…”

Residential Mobility During Pregnancy in the North of England

“…The importance of mobility in determining individual environmental exposures and in shaping the profile of the underlying population at risk has been acknowledged in many studies [2][3][4][5][6][7]. Nonetheless, epidemiological studies carried out at the ecological level, or using routinely collected health data, often struggle to account for the impact of mobility on exposure.…”

Estudios ecológicos

“…El mismo problema ocurre en los estudios con base en individuos; sin embargo, en los estudios ecoló-gicos se presenta la migración como problema adicional, es decir, la población afectada podría haber emigrado antes de la medición de la enfermedad, o algunos otros podrían haber inmigrado a la población estudiada, ocasionando un sesgo de selección. 18 El problema de inadecuada especificación del modelo de análisis es otro factor de discrepancias cuando se espera que los resultados de los estudios ecoló-gicos sean los mismos que de estudios individuales. Teóricamente, los resultados podrían ser los mismos cuando se emplean modelos lineales aditivos, sin tér-minos multiplicativos.…”