If the farmer performs the experiment naively, for example applying Fertilizer 1 to the high end of his field and Fertilizer 2 to the low end, he is probably introducing Drainage as a confounder. If he uses Fertilizer 1 one year and Fertilizer 2 the next year, he is probably introducing Weather as a confounder. In either case, he will get a biased comparison. The world that the farmer wants to know about is described by Model 2, where all plots receive the same
