The corrosive effect of various soils on coated and uncoated steel pipe was tested by using a dependent sampling plan. The data collected are summarized below, where d is the amount of corrosion. What is the purpose of using a dependent sampling plan in this study?
1) To compare the corrosion levels between coated and uncoated steel pipe
2) To determine the effect of different soils on the corrosion of steel pipe
3) To investigate the relationship between the amount of corrosion and the type of soil
4) To analyze the effectiveness of the coating on the steel pipe