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Last Edited: 3/18/08

What is a confounding factor?

Epidemiological studies search for the causes of diseases, based on associations with various risk factors that are measured in the study. In addition to the exposures that the study is investigating, there may be other factors that is associated with the exposure and independently affects the risk of developing the disease. If the prevalence of these other factors differ between groups being compared, they will distort the observed association between the disease and exposure under study.  These distorting factors are called confounding factors or variables.

A hypothetical example would be a study of coffee drinking and lung cancer. If coffee drinkers were also more likely to be cigarette smokers, and the study measured coffee drinking but not smoking, the results may seem to show that coffee drinking increases the risk of lung cancer, which may not be true. However, if a confounding factor (in this example, smoking) is recognized, adjustments can be made in the study design or data analysis so that the factor does not confound the study results.