For this assignment there are four given examples. From these examples identify which bias is occurring and support your reasoning.

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.