When research studies are performed by scientists, there are many variables that are carefully defined and quantified. A variable generally measures a characteristic of the survey or study that changes, like the intelligence level, gender or age of a person. The ability to control variables is an important key to the success of a research study; however, some variables are more difficult to control than others.


An extraneous variable is a factor which cannot be controlled. These variables may or may not influence the results of a survey or experiment.

Identification of Extraneous Variables

Six common types of variables exist, one of which is the extraneous variable. An extraneous variable is a factor which cannot be controlled. These variables may or may not influence the results of a survey or experiment. There are different ways to control extraneous variables, depending on what type of influence is desired on the results of a study. Extraneous variables add undesirable error to experiments, so decreasing or controlling the influence of these variables is a major goal.

Types of Extraneous Variables

Extraneous variables can be further defined by type. An extraneous variable that does not stay the same and varies with levels of the independent variable in a study is called a confounding variable. The goal of experiments is to simulate an environment where the only difference between various conditions is the difference in independent variables. This allows researchers to conclude that a manipulation is causing differences in a dependent variable.

If, however, there is another variable that changes with the independent variable, that confounding variable could be the underlying cause of any difference in the study, which could render the study or survey results inconclusive.

Dependent and Independent Variables

Dependent and independent variables are two key variable types used when designing studies.

An extraneous variable is related in the sense that independent variables are the factors in a research study that are measured, manipulated, or chosen by an experimenter to understand and determine their relationships to certain observed phenomena. In research studies, independent variables are manipulated or observed to understand the relationship to that of their dependent variable. A dependent variable shows the effects of changing or adding an independent variable.

Dangers of Studying Extraneous Variables

Extraneous variables can be dangerous to a study since they may damage its validity. This makes it impossible for researchers to know whether certain effects were caused by other variables, such as independent or moderator variables, or some unknown extraneous factor. Confounding variables are even more dangerous than simple extraneous variables, because they magnify the challenges of achieving the validity of experimental results.

Compensating for Extraneous Variables

While there are several other types of variables used in research studies, extraneous variables cause problems because they cannot be controlled. This must be considered when study results are interpreted. One way to compensate for extraneous variables is to use a tool called random assignment.

When subjects in a study are randomly assigned to different groups, then on average the two groups will be equal in age, intelligence or whatever factors are descriptive of the groups under study. Although this does not decrease the error amount that occurs because of the extraneous variables, it will at least equalize the error between the groups under study.