Bias in Epidemiologic Research

For the following study descriptions, describe potential bias(es) most likely to be present. Indicate how the bias would affect the measure of association (would it be underestimated, overestimated or unchanged):

1. A cohort study was designed to look at the association of pesticides and the occurrence of childhood leukemia. Children living in rural areas were followed and leukemia rates were compared between those children living in areas with high and low pesticide exposure. Pesticide exposure was considered high if it was over 20 "units" and low if it was under 20 "units." The method used to quantify pesticide exposure was accurate to within +/- 12 units.
2. Persons diagnosed as HIV positive or negative were interviewed about their number of lifetime sexual partners. The interviews occurred after they were told their HIV status.
3. A cohort study looked at the association of exposure to PCBs in the workplace and the occurrence of cancer over 20 years. At the 10-year follow-up, 15% of the original participants could not be located to assess their disease status.
4. In a clinical trial of a new drug, the intervention group received the medicine and outcomes were assessed by a doctor working for the pharmaceutical company. The placebo group received the placebo and outcomes were assessed by a doctor from the local community hospital.