Are people already at increased risk for disease more likely to be exposed to the risk factor of interest? Does closer observation of people with a disease lead to a false association? In retrospective studies, do people with a disease recall prior exposures more (or less) that healthier people? Are research interviewers a source of biased data collection? Confounding is an existential threat in biomedical research; here a second factor, which is associated with both the disease and the risk factor being studied, is an actual cause of the disease. If studies cannot fully control for the effect of the second risk factor, residual confounding will bias the risk estimate. Who participates and doesn’t participate in research is another source of bias. How diseases and risk factors are classified and categorized may introduce bias, and changing defined categories is yet another source of bias.