A second potential source of the discrepancies may come from how the exposure or lack of exposure to environmental tobacco smoke is determined. Studies have shown that people can recall recent exposure very well but that remembering the duration and degree of distant exposure (such as whether their grandparents or baby-sitter smoked) is difficult. Yet one study examined this issue and found that women tended to underestimate their exposure, an effect which would decrease the observed risk. Thus, the information used in these studies may be inaccurate which could influence the reported breast cancer risk association. More work is needed to resolve these issues
