Why randomize? Fertilizer and crop yield
A supplier reports that fields treated with a new fertilizer produced higher yields than untreated fields. But farmers decided where to apply it, and they naturally put it on their most promising plots.
Explain why the yield gap cannot establish that the fertilizer raises yield, and design a trial that can. Why is randomization the key ingredient?
Your answer
This one is open-ended. Work it through, then check your reasoning against the full solution.