Suppose that different amounts of fertilizer had been used in different fields that were involved in the wheat trials. (a) Explain how the use of different amounts of fertilizer confounds the...

Suppose that different amounts of fertilizer had been used in different fields that were involved in the wheat trials.

(a) Explain how the use of different amounts of fertilizer confounds the experimental analysis in the absence of randomization.

(b) Considering regression models if the amounts of fertilizer were known, how could an analyst adjust for this confounding effect?