Choose one of the following that could have biased the results: selection bias (direction: positive or confounding (direction: positive or information bias (direction: positive or and random error.1. the authors of a cross-sectional study hypothesized that lack of regular exercise is associated with obesity in children. their study of 12 children in michigan, however, failed to show a statistically significant association between exercise habits and obesity (or = 1.9, p = 0.11).2. in a case-control study of the relationship between intravenous drug use and hiv infection, the investigators discovered after the study that the case group had tended to underreport their drug use due to fears arising from previous convictions for drug-related violations. underreporting was not a problem in the control group, however, which was not under medical supervision at the time.3. a prospective cohort study followed 8,542 women for 10 years to determine if alcohol consumption increased the risk of breast cancer. no statistically significant association was found. twelve thousand women had been enrolled in the study at its initiation, but 3,458 had withdrawn during the 10-year period. a follow-up of spouses, friends, and relatives of the women who withdrew revealed that more than 60% of the withdrawals had developed breast cancer and that nearly 84% were regular drinkers.4. a large epidemiologic study found that elderly adults with dementia were more likely to develop liver cancer than those without dementia. the investigators, however, could offer no plausible biological mechanism for the association between dementia and liver cancer. many of the subjects reported heavy drinking, however.