Susan is conducting an experiment to see if plants will grow better with application of fertilizer. She separates 50 plants into two groups of 25. One group receives a liquid fertilizer when watered every other day, and the other group receives only water on the same days. The plants are kept in a greenhouse with constant and equal amounts of sunlight, and a constant temperature. She measures the plants once a week for 12 weeks. At the end of 12 weeks, the plants with the fertilizer grew an average of 9 inches, and the ones that were not given the fertilizer grew an average of 5 inches. When referencing the above scenario, why did Susan average the resulting heights of the plants