Effects of Audio-visual Education on Grades A group of educators wants to determine the effectiveness of multimedia tools in raising students' grades. So they gave the students an option to choose a class with audio-visual tools or one without any.

Then they conducted a test to check how well the students retain their lessons and compared the results of both the groups. Let's assume the group that chose the audio-visual class received higher grades.

Does that show that the audio-visual teaching aids work better? If not, explain why not and suggest a confounding variable.