Control Selection Strategies in Case-Control Studies of Childhood Diseases

Abstract

To address concerns regarding the representativeness of controls in case-control studies, two selection strategies were evaluated in a study of childhood leukemia, which commenced in California in 1995. The authors selected two controls per case: one from among children identified by using computerized birth records and located successfully, the other from a roster of friends; both were matched on demographic factors. Sixty-four birth certificate-friend control pairs were enrolled (n = 128). Additionally, 192 "ideal" controls were selected without tracing from the birth records. Data on parental ages, parental education, mother's reproductive history, and birth weight were obtained from the birth certificates of all 320 subjects. For all variables except birth weight, the differences between birth certificate and ideal controls were smaller than those between friend and ideal controls. None of the differences between birth certificate and ideal controls was significant, whereas two factors were significantly different between friend and ideal controls. These findings suggest that friend controls may be less representative than birth certificate controls. Despite difficulty in tracing and a seemingly low participation rate (49.0% for 560 enrolled birth certificate controls), using birth records to recruit controls appears to provide a representative sample of children and an opportunity to assess the representativeness of controls.

Ethnic Difference in Daycare Attendance, Early Infections, and Risk of Childhood Acute Lymphoblastic Leukemia

“…An earlier evaluation indicated that controls enrolled in the NCCLS were comparable to ideal controls who would have been enrolled under optimal circumstances with respect to parental ages, maternal history of fetal loss, birth weight, birth order, and time since last live birth (9). However, cases and controls included in the present analysis were significantly different in annual household income and maternal education, two variables that reflect socioeconomic status and may also affect daycare attendance and early infections.…”

“…For cases diagnosed on or after December 1, 1999, the case-control ratio is 1:2. A detailed protocol for control selection has been reported elsewhere (9).…”

Mode of Delivery and Risk of Childhood Leukemia

“…The California Childhood Leukemia Study (CCLS) is described in detail elsewhere (21) Briefly, the CCLS is an ongoing case-control study in California, which commenced in 1995. The study began in the 17 counties surrounding the San Francisco Bay Area, and now (up to 2008) encompasses most of California through collaboration with 35 pediatric oncology centers in the state, allowing for rapid case ascertainment, usually within 72 hours from diagnosis.…”

Parental social contact in the work place and the risk of childhood acute lymphoblastic leukaemia

“…A previous comparison of the study with the California Cancer Registry indicated that 88% of childhood leukaemias were included in this study. NCCLS controls selected from birth certificates also appear to be representative of the population of the study area (Ma et al, 2004).…”

“…A detailed description of control selection has been published previously (Chang et al, 2006). The participating controls of the NCCLS are representative of the sampled population in parental age, parental education, and mother's reproductive history, which indicates a reduced potential for selection bias (Ma et al, 2004).…”