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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):
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.
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.
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.
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.
Course Text: Essentials of Epidemiology in Public Health
Chapter 6, "Overview of Epidemiologic Study Designs
Chapter 6 presents an overview of analytic study designs that explore associations between exposures and outcomes. These study designs can be categorized as either experimental (randomized controlled trial or community trial) or observational (ecologic, cross-sectional, case-control, or cohort studies).
Chapter 10, "Bias"
Chapter 10 provides an overview of the concept of bias, which is defined as a systematic error that can occur in the design, implementation, or analysis of a study and lead to an invalid measure of association. The reading emphasizes two types of bias: selection bias and observation bias (also known as information bias). It describes these biases and addresses ways in which they can be minimized.
Optional Activities
Study Types in Epidemiology
http://www.nwcphp.org/training/courses/study-types-in-epi
This module from the Northwest Center for Public Health Practice examines different types of study designs. It focuses on the two main categories of descriptive and analytic studies and then identifies the different study types associated with each category. This module will give you a good overview of the various types of studies used in epidemiology and lay a solid foundation for the upcoming weeks.
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