Questions to Consider When Reading a Paper or Listening to a Presentation
- Given the specific conditions of the study, could bias have occurred? Which types?
- Is bias actually present?
- In which direction is the bias? Toward the null or away from the null?
- Are consequences of the bias likely to be large enough to distort the measure of association in an important way?
A study reported a risk ratio of 2.5 for the association between current or past smoking and coronary heart disease. A reviewer who critiqued the study pointed out that there could have been substantial non-differential misclassification of smoking exposure, and she argued that there may have been no association, i.e., that the RR=2.5 could have been due entirely to misclassification.
A case control study was conducted to determine if using antihistamines around the time of conception increased the risk of birth defects in the offspring. No personal interviews were conducted regarding subjects' antihistamine use. Instead, women were considered exposed if computerized pharmacy records from their HMO indicated that they had filled at least one prescription for antihistamines within 500 days before the birth of the child. This method of ascertaining exposure has the advantage of avoiding recall bias, but could it introduce any other type of bias?
A prospective cohort study was conducted to determine the risk of heart attack among men with varying levels of baldness. Third-year residents in dermatology conducted visual baldness assessments at the start of the study (before any heart attacks occurred). Four levels of baldness were coded: none, minimal, moderate, and severe. The follow-up rate was close to 100%. Could any of the following types of bias have occurred in this study?
- Recall bias of exposure information
- Differential misclassification of exposure
- Non-differential misclassification of exposure
- Selection bias