Kaley is trying to determine how music affects the growth of plants. She hypothesizes that classical music will cause a plant to grow at a higher rate than no music or rap music. She grows 9 tomato plants in similar lighting, temperature, and soil. Three are exposed to classical music 24 hours a day, three are exposed to rap music 24 hours a day, and three are grown in complete silence. Kaley measures the growth of the plants every day and finds that the plants all seem to have the same rate of growth. What is missing from Kaley’s scientific investigation?