Attenuation of social class and reproductive risk factor associations for Hodgkin lymphoma due to selection bias in controls

Objective: Hodgkin lymphoma (HL) risk has been linked with higher social class and lower parity, but our prior population-based case-control study in adult women had unexpected null findings for these variables. Because subject participation was 87% for cases but 65% for random digit-dialing (RDD) controls, we examined representativeness of our controls and the impact of detected bias on prior results. Methods: Using data from RDD enumeration, abbreviated interviews with nonparticipating controls, and the US census, we compared participating and nonparticipating RDD controls across several age groups and then recomputed odds ratios for risk factor associations adjusted for bias. Results: The 325 RDD control participants were younger, more likely to be white, better educated, and of lower birth order and lower parity than the nonparticipants. Adjustment of odds ratios for bias strengthened previously null findings for education and for parity, breast-feeding and miscarriages in young adult women; these latter changes eliminated previously apparent age modification of risks. Conclusions: Selection bias in female RDD controls resulted from differential participation by socioeconomic factors, varied with age, and produced underestimations of several associations in young women, including reproductive factors. Thus, our prior conclusions of etiologic irrelevance for some study variables may have been inaccurate.