The situation in which the effect of a controlled variable is inextricably mixed with that of another, uncontrolled variable. For example, if we select children from a schoolyard and form experimental groups according to their heights, the groups would be very likely to differ in age as well.
When the effects of two or more factors cannot be separated, e.g. a study recruiting old men and young women confounds age and gender effect. A well-designed study should seek to minimise such effects.
A term used to describe a study observation that has one or more nuisance variables present that may lead to incorrect interpretations.