Suppose you would like to evaluate whether there is a causal impact of high school sports participation on one’s likelihood of graduating from college. This is a tricky question to evaluate because there is likely selection into high school sports participation. Now suppose the state of Missouri has recently passed a legislation mandating that only high school students above a certain height (which differs by gender) will be allowed to participate in school sports. No student who is shorter than the gender-specific mandated height is allowed to