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Why is sample selection based on the dependent variable problematic? Why is sample selection based on the independent variable not problematic? Consider the following function. $saving=\beta_0+\beta_1income+\beta_2age+\beta_3size+u$ If our data set was based on a survey of people over 35 years of age, thereby leaving us with a nonrandom sample of all adults, the coefficient estimators are still unbiased and consistent. Consider another function $wealth=\beta_0+\beta_1educ+\beta_2exper+\beta_3age+u$. Suppose that only people with wealth below $250,000 are included in the sample. This will result in biased and inconsistent estimators of the parameters. Why is that the case? Can you explain intuitively?

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