A confounding variable is a variable that affects the outcome of the test but is unknown to/ unobserved by the researchers. Example: You measure the effect of

A confounding variable is a variable that affects the outcome of the test but is unknown to/ unobserved by the researchers. Example: You measure the effect of alcoholic beverages on general health. You create two groups, one group consists of the beer drinkers, another group gets the wine drinkers. Result: drinking wine correlates with better health, drinking beer does not. The confounding variable in this case would be class affiliation: working class members are more likely to drink beer than wine and are more probable to eat less healthy food and have less healthy jobs. By defining the groups solely by choice of beverage, the researchers have unwillingly split by the confounding variable of class affiliation.