Recruiting control subjects who are representative of the population from which the cases are drawn is a challenge in case–control studies. This paper examines the performance of random digit dialling (RDD) in obtaining a control sample, and the sample's representativeness of the population with respect to socio-economic status. The study subjects were recruited from 2003 to 2006 for a national, population-based case–control study investigating causes of acute lymphoblastic leukaemia (ALL) in children <15 years of age in Australia. Control families' addresses were linked to Australian Bureau of Statistics Census 2006 Collection Districts and thus to Socio-Economic Indexes for Area scores, which are area-based measures of socio-economic status. These scores were compared with those of all collection districts where families lived. We estimate that 55% of eligible families in the RDD sample agreed to participate in the study. Participation was directly related to socio-economic status with those of highest economic status most likely to participate. Completeness of participation in the components of data collection was similarly related to socio-economic status. This evidence of selection according to socio-economic status indicates that there may also be selection with respect to other factors potentially important in the aetiology of ALL.
Paediatric and Perinatal Epidemiology Vol. 24, Issue 3, p. 293-302