Extraneous Variables are undesirable variables that influence the relationship between the variables that an experimenter is examining.

Another way to think of this, is that these are variables the influence the outcome of an experiment, though they are not the variables that are actually of interest. These variables are undesirable because they add error to an experiment.

A major goal in research design is to decrease or control the influence of extraneous variables as much as possible (Hall, 2011). Extraneous variables that vary with the levels of the independent variable are the most dangerous type in terms of challenging the validity of experimental results.

These types of extraneous variables have a special name, confounding variables. Any experiment that involves a comparison of men and women is inherently plagued with confounding variables, the most commonly cited of which is that the social environment for males and females is very different.

An example of extraneous variable is if a study focused on the effect of relaxation therapy on the perception of incisional pain, the researchers would have to control the extraneous variables, such as type of surgical incision and time, amount, and type of pain medication administered after surgery, to prevent their influence on the patient's perception of pain.

Selecting only patients with abdominal incisions who are hospitalized and receiving only one type of pain medication intravenously after surgery would control some of these extraneous variables. Controlling extraneous variables enables researchers to determine the effects of an intervention or treatment on study outcomes more accurately

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