Confounding:  A procedure observed in some factorial designs in which it is impossible to differentiate between some main effects or interactions, on the basis of the particular design used. Essentially the contrast which measures one of the effects is exactly the same as the contrast which measures the other. The two effects are usually referred to as aliases. The term is also used in observational studies to highlight that a measured association found between an exposure and an outcome may not represent a causal effect because there may exist variables which are related with exposure and outcome both.