Nov 18

Application: Ordinary Least Squares estimator

Application: Ordinary Least Squares estimator

Generalized Pythagoras theorem

Exercise 1. Let P be a projector and denote Q=I-P. Then \Vert x\Vert^2=\Vert Px\Vert^2+\Vert Qx\Vert^2.

Proof. By the scalar product properties

\Vert x\Vert^2=\Vert Px+Qx\Vert^2=\Vert Px\Vert^2+2(Px)\cdot (Qx)+\Vert Qx\Vert^2.

P is symmetric and idempotent, so

(Px)\cdot (Qx)=(Px)\cdot[(I-P)x]=x\cdot[(P-P^2)x]=0.

This proves the statement.

Ordinary Least Squares (OLS) estimator derivation

Problem statement. A vector y\in R^n (the dependent vector) and vectors x^{(1)},...,x^{(k)}\in R^n (independent vectors or regressors) are given. The OLS estimator is defined as that vector \beta \in R^k which minimizes the total sum of squares TSS=\sum_{i=1}^n(y_i-x^{(1)}\beta_1-...-x^{(k)}\beta_k)^2.

Denoting X=(x^{(1)},...,x^{(k)}), we see that TSS=\Vert y-X\beta\Vert^2 and that finding the OLS estimator means approximating y with vectors from the image \text{Img}X. x^{(1)},...,x^{(k)} should be linearly independent, otherwise the solution will not be unique.

Assumption. x^{(1)},...,x^{(k)} are linearly independent. This, in particular, implies that k\leq n.

Exercise 2. Show that the OLS estimator is

(2) \beta=(X^TX)^{-1}X^Ty.

Proof. By Exercise 1 we can use P=X(X^TX)^{-1}X^T. Since X\beta belongs to the image of P, P doesn't change it: X\beta=PX\beta. Denoting also Q=I-P we have

\Vert y-X\beta\Vert^2=\Vert y-Py+Py-X\beta\Vert^2

=\Vert Qy+P(y-X\beta)\Vert^2 (by Exercise 1)

=\Vert Qy\Vert^2+\Vert P(y-X\beta)\Vert^2.

This shows that \Vert Qy\Vert^2 is a lower bound for \Vert y-X\beta\Vert^2. This lower bound is achieved when the second term is made zero. From

P(y-X\beta)=Py-X\beta =X(X^TX)^{-1}X^Ty-X\beta=X[(X^TX)^{-1}X^Ty-\beta]

we see that the second term is zero if \beta satisfies (2).

Usually the above derivation is applied to the dependent vector of the form y=X\beta+e where e is a random vector with mean zero. But it holds without this assumption. See also simplified derivation of the OLS estimator.

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