In many cases, extraneous variables are controlled for by the experimenter.
In the case of participant variables, the experiment might select participants that are the same in background and temperament to ensure that these factors do not interfere with the results.
If, however, a variable cannot be controlled for, it becomes what is known as a confounding variable.
This type of variable can have an impact on the dependent variable, which can make it difficult to determine if the results are due to the influence of the independent variable, the confounding variable or an interaction of the two.
Confounding variable examples - Weather or a dog bark in the distance.
(things that aren't controlled by the independent variable)
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