Why randomize? Fertilizer and crop yield

Question

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?

Accepted Answer

Why the gap is not enough. Farmers chose where to apply the fertilizer, favoring their best land, better soil, drainage, and sun exposure. Land quality is a confounder: good plots yield more and were the ones fertilized, so the fertilizer inherits credit that belongs to the soil. The treated and untreated fields were not comparable to begin with. A trial that can prove causation. Take a set of plots and randomly assign each to fertilized or unfertilized, ideally splitting fields into paired sub-plots to balance local conditions, then compare yields. This is the classic randomized agricultural…

Why randomize? Fertilizer and crop yield

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