An experimenter tests the hypothesis that physical exercise improves mood. subjects in the experimental group participate on mondays and tuesdays and those in the control group on wednesdays and thursdays. which is the confounding variable?

The confounding variable in this situation is the disparity between the days of the week on which individuals performed physical exercise to improve their mood.

Perhaps, individuals generally have a lower and more supressed mood on Mondays versus Thursdays causing the results to be skewed as they failed to account for the confounding variable of different dates.