Confounding variable
Confounding variables are outside the control of the researchers, and may influence outcomes of an intervention. These variables can minimize or exaggerate the observed effect of an intervention.
For example, Professor X used Textbook A in her class while Professor Y used Textbook B, and the students of both classes were given the same exam to test their knowledge. The independent variables (the textbooks and the professors’ teaching effectiveness) would be confounded. By looking at the data, there would be no way to tell whether any differences in exam scores between the two classes were caused by the different textbooks, the different professors, or a combination of both.