A student wants to compare the effectiveness of the laundry detergents Tide® and Wisk® Deep Clean. He washes some stained towels with Tide in a Kenmore® washing machine and washes other stained towels with Wisk Deep Clean in a Samsung® washing machine. After inspecting the washed towels, the student concludes that Tide is more effective than Wisk Deep Clean.
a. Describe the problems with the student’s study. Include in your description at least one possible lurking or confounding variable and identify which type it is.
b. Redesign the study so that it is a well-designed experiment.