Item CV016003: To determine the effect of water temperature on getting stains out of shirts, it is important for all shirts to be washed for the same length of time because if they weren't, it wouldn't be possible to find out the effect of temperature.
To conserve energy, a college student wants to find out the lowest water temperature and the shortest length of time that is needed to wash his clothes and get them clean. First he tests the effect of water temperature. He washes one shirt in hot water, one shirt in warm water, and one shirt in cold water. The shirts are identical and have identical stains. He uses the same detergent and washes the shirts for the same length of time. Why is it important to wash the shirts for the same length of time?

Distribution of Responses Percent of students responding correctly
 