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

This conclusion is not valid because BOTH SOIL BRANDS WERE NOT SUBJECTED TO THE SAME CONDITIONS. 
Carlisle applied brand A to the part of the yard that received full sunlight while brand B was applied to the area of the yard which received partial sunlight. This implies that the soils were subjected to different conditions and this ought not to be so, since sunlight is a very important factor, which the soils need.

Answer 2

His conclusion is not valid because his dependent variables weren't the same. When testing out an independent variable, which is the soil in this case, all the other factors of the experiment has to remain the same in order to provide a correct conclusion. Since Carlisle was testing out two different types of soils, he had to set all the other conditions of the plants exactly the same, including the amount of lights that the plants receive. Brand B received full sunlight whereas brand A received less. This could definitely have impaired the results of this experiment, therefore, making it invalid.