Question

Carlisle is testing different types of planting soil to determine which type is most effective in growing blueberry bushes. He purchases 2 different brands of planting soil from a local store. Carlisle 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. He waters both areas daily. At the end of the study, Carlisle concludes Brand A is more effective in growing blueberry bushes. Why is his conclusion not valid?

Answer 1

There is a confounding variable - amount of sunlight each plant receives.
He must eliminate this variable by putting plants in the same location (and thus amount of sunlight).

Otherwise how could be tell whether the brands were at work, or whether it was simply because the plant got more sunlight?