Sarah is testing different types of planting soil to determine which type is most effective in growing blueberry bushes. She purchases 2 different brands of planting soil from a local store. Sarah applies Brand A to an area of the yard that receives full sunlight and Brand B to another area of the yard that is in partial shade. She waters both areas daily. At the end of the study, Sarah concludes Brand A is more effective in growing blueberry bushes. Why is her conclusion not valid?