First, is the instrumental variable Z independent of the confounder? The randomization of Z ensures that the answer is yes. (As we saw in Chapter 4, randomization is a great way to make sure that a variable isn’t affected by any confounders.) Is there any direct path from Z to Y? Common sense says that there is no way that receiving a particular random number (Z) would affect cholesterol (Y), so the answer is no. Finally, is there a strong association between Z and X? This time the data themselves should be consulted, and the answer again is yes. We must always ask the above three questions
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