The manager of a public swimming pool wants to compare the effectiveness of two laundry detergents, Detergent A and Detergent B, in cleaning towels that are used daily. As each dirty towel is turned in, it is placed into the only washing machine on the premises. When the washing machine has 20 towels in it, the manager flips a coin to determine which detergent will be used for that load. The cleanliness of the load of towels is rated on a scale of 1 to 10 by a person who does not know which detergent was used. The manager continues this experiment for many days.
20 towels per wash indicates which component of a well-designed experiment?