Confounding factors or variables are variable other than the risk factor, for which cases and control groups differ, age is the most common example. The possiblity of unknown confounding variables makes it difficult to state categorically that a factor is a cause. Confounding should be addressed either in the design stage or with analytical techniques. In the design stage, confounding can be controlled for by restriction or matching. Many researchers prefer to handle it in the analysis phase with analytical techniques like logistic regression or by stratification with Mantel-Haenszel approaches.