So he and Gerber began to read more generally about the origins of field experiments in other areas. The term hinted at the history: the earliest randomized trials grew out of searches for fertilizer compounds conducted by nineteenth-century researchers for the nascent chemical industry. ... Each season, scientists at the Rothamsted Agricultural Experimentation Station in England would take a blend of compounds such as phosphate and nitrogen salts, alter the ratio of the chemicals, and sprinkle it over plots of rye, wheat, and potato planted in the clay soil of the estate north of London. One year’s plant growth would be compared with the next, and the difference was recorded as an index of fertility for each chemical mixture. When the pipe-smoking mathematician R. A. Fisher arrived in 1919 and examined ninety years of experiments, he realized that the weather probably had had more to do with the variations in growth than the chemical blend. Even though Rothamsted researchers tried to discount for the volume of rain in a given season, there were many other things that varied unpredictably and even imperceptibly from year to year, like soil quality or sun or insect activity. Fisher redrew the experiment so that different chemical ratios could be compared with one another simultaneously. He split existing plots into many small slivers and then randomly assigned them different types and doses of fertilizer that could be dispensed at the same time. The size and proximity of the plots ensured that, beyond the varying fertilizer treatments, they would all experience the same external factors.
“The Victory Lab: The Secret Science of Winning Campaigns”, Sasha Issenberg