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STATISTICS 512 TECHNIQUES OF MATHEMATICS FOR ...

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24<br />

3. Orthogonality; Gram-Schmidt method;<br />

QR-decomposition<br />

• Hat matrix: Consider a regression model y =<br />

Xθ + with X × of full rank . We will later<br />

show that the LSEs are<br />

ˆθ = ³ X 0 X´−1<br />

X 0 y<br />

so that the estimate of [y] =Xθ is ŷ = Xˆθ =<br />

Hy, where<br />

H × = X ³ X 0 X´−1<br />

X<br />

0<br />

is the “hat” matrix - it “places the hat on y”.<br />

Properties:<br />

H = H 0 = H 2 (“idempotent”)<br />

HX = X<br />

(I − H)X = 0<br />

(I − H) 2 = (I − H)<br />

H(I − H) = 0

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