3.29 For the simple linear regression model, show that the elements of the hat matrix are \begin{align*} h_{ij} &= \frac{1}{n} + \frac{(x_i - \bar{x})(x_j - \bar{x})}{S_{xx}}\\ \text{and } h_{ii} &= \frac{1}{n} + \frac{(x_i - \bar{x})^2}{S_{xx}} \end{align*} Discuss the behavior of these quantities as $x_i$ moves farther from $\bar{x}$,
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The hat matrix, denoted as H, is defined as H = X(X^T X)^(-1) X^T, where X is the design matrix of the predictor variable x, and X^T represents the transpose of X. Show more…
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Problem 4.4: Consider the following simple regression model for which εi ~ N(0,σ^2): y1 = β0 + 0.5β1 + ε1 y2 = β0 – β1 + ε2 y3 = β0 + 0.5β1 + ε3 (a) Find the least squares estimates using vectors and matrices. (b) Find the covariance matrix of β̂. (c) Find the hat matrix H. Verify that the sum of the diagonal elements of the hat matrix is equal to 2 (Σⁿᵢ₌₁ hii = p + 1).
Dominador T.
a) H is symmetric. (b) 0 < hii < 1, where hii is the ith diagonal entry of the hat matrix. HINT: First show that hii = hii. c) The off-diagonals of the hat matrix are found by the formula (x - x)(x - x) SXX / n. d) Finally, the text states that "There is a small amount of correlation present in standardized residuals, even if the errors are independent." Comment on when the covariances of the residuals are close to zero, for a fixed sample size. Why does it make sense that the covariances are close to zero in those situations?
Adi S.
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