Any characteristic that makes it difficult to compare two or more distinct groups in an epidemiologic study. Confounding factors can mask a health effect so that the relationship of the effect and the exposure is not recognized. They can also make it appear as though there is an effect when, in fact, none exists.

Variables other than those being tested which can affect the incidence or degree of a parameter being measured.

Factors which may influence experimental results but which are not necessarily themselves key factors or being studied