A study investigates the effect of three different toothpaste brands (P, Q, R) and water on plaque removal. $72$ artificial teeth are randomly assigned to one of four treatments: toothpaste P, toothpaste Q, toothpaste R, or water. Each tooth is brushed in a mechanical device for $10$ minutes, repeated four times (for a total of $40$ minutes). The researcher measuring the plaque removal is blinded to the treatment methods. The percentage of plaque removed is recorded. Researchers found that P and Q removed more plaque than R or water. State a factor that could affect the response variable that is fixed at a set level.