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The difference is that the first regression is unbiased only if you can assume that high school GPA and ACT score are orthogonal on each other $cov(x,z)=0$ where $x$ is shortcut for high school GPA and $z$ for ACT score. Or if you can assume the second variable ATC score does not affect the dependent variable at all $\beta_2=0$. This is because in simple ...


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I have spent three days thinking about how to answer your question. I still haven't really decided how it should be answered, so this answer is more in the way of a set of observations. I can readily imagine a formal treatment of general linear models or limited dependent variables models could do better than this answer. The first thing I would observe ...


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