A Confounding variable is a variable that produces unexpected changes in the dependent variable and therefore interferes with interpreting the capacity of an independent variable to produce or explain changes in a dependent variable.
Example: During a study of anxiety that includes measures of anxiety and stress, some participants watch a documentary about the treatment of anxiety and some do not. Documentary-watching may confound the results if watching the program influenced the scores on the measures of anxiety and stress. Similarly, some participants may be exposed to a source of stress in their environment but others are not, which could interfere with interpreting the results.
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