1 Department of Epidemiology and Public Health, Yale University School of Medicine, New Haven, CT.
2 Division of Public Health Biology and Epidemiology, University of California, Berkeley, CA.
3 Public Health Institute, Oakland, CA.
4 Environmental Health Investigations Branch, California Department of Health Services, Oakland, CA.
Received for publication February 23, 2004; accepted for publication March 3, 2004.
We appreciate the thoughtful commentary by Ross et al. (1) on our manuscript in this issue of the Journal (2). We are encouraged by their plan to assess the feasibility of obtaining birth certificates for control selection nationwide.
The focus of our manuscript was on comparing birth certificate controls and friend controls with reference to a population sample of births. We used parental ages, parental education, mothers reproductive history, and birth weight to make these comparisons. Ross et al. suggested that it would be interesting to see how comparable the different control groups would be with respect to additional variables, including maternal cigarette smoking, alcohol consumption, and method of delivery, for example. Unfortunately, California birth certificates do not include information on cigarette smoking or alcohol consumption. We had not considered method of delivery (vaginal vs. cesarean section) a factor of etiologic significance, and we did not observe notable differences when we compared this factor between the control groups. In response to a comment from one reviewer of our manuscript, we evaluated two variables related to prenatal carethe month in which prenatal care was started and the total number of prenatal visits. The results were consistent with what was observed for the variables already included in the manuscript.
For the subset of the 64 cases and their matched birth certificate controls, we also compared the characteristics of potential controls who were initially selected for one of the cases but did not participate with those of the actual participating controls, that is, the replacement control for the nonparticipating controls. The characteristics we evaluated included not only the variables from the birth certificates but also neighborhood socioeconomic characteristics based on the 1990 US Census block group, including median family income, the percentage of the population older than age 18 years with a high school degree, and the percentage of the population older than age 25 years with a bachelors degree. We obtained the neighborhood data by linking maternal residence addresses listed on birth certificates to census block groups using a geographic information system. The results we observed were reassuring since the nonparticipating controls and their replacements were comparable with respect to all of the characteristics evaluated.
The assessments we have conducted to date are based on a relatively small sample. We plan to conduct similar evaluations on a much larger scale as the study continues. The Northern California Childhood Leukemia Study was recently funded for an additional 5 years of case ascertainment. We expect to enroll over 1,100 cases and a larger number of controls throughout the study. We hope that additional information from our study, the study planned by Ross et al., and possibly similar studies by other investigators will help further assess the theoretical and practical advantages and disadvantages of control selection using birth records in case-control studies of childhood diseases.
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