The insured in the NHIS are healthier for all sorts of reasons, including, perhaps, the causal effects of insurance. But the insured are also healthier because they are more educated, among other things. To see why this matters, imagine a world in which the causal effect of insurance is zero (that is, κ=0). Even in such a world, we should expect insured NHIS respondents to be healthier, simply because they are more educated, richer, and so on. This positive selection bias runs counter to the negative selection bias we imagined in the parable of frail, insured Khuzdar and hearty, uninsured
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