International Journal of Epidemiology, Vol 28, 1130-1133, Copyright © 1999 by International Epidemiological Association
A Agudo and CA Gonzalez
BACKGROUND: The problem of control selection was considered in a
population-based case-control study on pleural mesothelioma in Spain.
Random sampling from the population was discarded because of potential
selection bias due to low participation. Selection of hospital controls by
matching them to cases by hospital seemed unsuitable for investigating
environmental exposures, as the choice of hospital may be related to the
place of residence; controlling for residence may avoid bias but could
produce overmatching. METHODS: A three-step procedure was proposed. First,
a random sample of primary controls from the population census of the
province of Barcelona was obtained. Second, the hospital closest to the
residence of the primary control was identified as the control hospital.
Third a secondary control was chosen among patients admitted to the
hospital matched to the primary control by sex, age and municipality.
RESULTS: An overall participation rate of 85% was achieved. The hospital
control group showed a distribution of residences similar to that of the
general population, and independent of the distribution of cases.
CONCLUSIONS: This procedure may be considered as an alternative for control
selection when studying environmental factors or, generally, when matching
cases and controls by hospital is to be avoided. Its validity was assessed
according to the principles of comparability with cases regarding the study
base, accuracy of information, deconfounding and efficiency.
Secondary matching: a method for selecting controls in case-control studies on environmental risk factors
Institute of Epidemiology and Clinical Research, Mataro, Spain. agudo@csm.scs.es
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