Mark wants to find the best laundry detergent to clean his workout clothes. He usually buys the generic store brand for his clothes but he thinks there might be a better brand to use. Mark decides he is going to test this out. He is going to compare Tide, Persil, and Gain with his generic store brand detergent that he usually buys. After two weeks he divides his dirty workout clothes into four piles with the same amount of clothing in each and washes each pile with one of the detergents he chooses. He repeats this process by washing his clothes with the different detergents two more times in four weeks. After the washes, he compares the smell and appearance of the clothes.