(1) there is no arrow between Miasma and Water Company (the two are independent), and (2) there is an arrow between Water Company and Water Purity. Left unstated by Snow, but equally important, is a third assumption: (3) the absence of a direct arrow from Water Company to Cholera, which is fairly obvious to us today because we know the water companies were not delivering cholera to their customers by some alternate route. FIGURE 7.8. Diagram for cholera after introduction of an instrumental variable. A variable that satisfies these three properties is today called an instrumental variable.
