title: Secondary matching: a method for selecting controls in case-control studies on environmental risk factors.
abstract: 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.
authors.affiliation: Institute of Epidemiology and Clinical Research, Mataró, Spain. agudo@csm.scs.es
authors.forename: A
authors.initials: A
authors.lastname: Agudo
authors.forename: C A
authors.initials: CA
authors.lastname: González
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D016022:Case-Control Studies; D006801:Humans; D008654:Mesothelioma / Q000453:epidemiology; D008875:Middle Aged; D015337:Multicenter Studies as Topic; D018579:Patient Selection; D010997:Pleural Neoplasms / Q000453:epidemiology; D011897:Random Allocation; D015203:Reproducibility of Results; D012107:Research Design; D012307:Risk Factors; D013030:Spain / Q000453:epidemiology