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< 16.1 Least Squares Regression Problem Statement | Contents | 16.3 Least Squares Regression Derivation (Multivariable Calculus) >

Least Squares Regression Derivation (Linear Algebra)

First, we enumerate the estimation of the data at each data point \(x_i\)

\[\begin{eqnarray*} &&\hat{y}(x_1) = {\alpha}_1 f_1(x_1) + {\alpha}_2 f_2(x_1) + \cdots + {\alpha}_n f_n(x_1), \\ &&\hat{y}(x_2) = {\alpha}_1 f_1(x_2) + {\alpha}_2 f_2(x_2) + \cdots + {\alpha}_n f_n(x_2), \\ &&\qquad\qquad \qquad \qquad \quad \cdots\\ &&\hat{y}(x_m) = {\alpha}_1 f_1(x_m) + {\alpha}_2 f_2(x_m) + \cdots + {\alpha}_n f_n(x_m).\end{eqnarray*}\]

Let \(X\in {\Bbb R}^n\) be a column vector such that the \(i\)-th element of \(X\) contains the value of the \(i\)-th \(x\)-data point, \(x_i, \hat{Y}\) be a column vector with elements, \(\hat{Y}_i = \hat{y}(x_i), {\beta}\) be a column vector such that \({\beta}_i = {\alpha}_i, F_i(x)\) be a function that returns a column vector of \(f_i(x)\) computed on every element of \(x\), and \(A\) be an \(m \times n\) matrix such that the \(i\)-th column of \(A\) is \(F_i(x)\). Given this notation, the previous system of equations becomes \(\hat{Y} = A{\beta}\).

Now if \(Y\) is a column vector such that \(Y_i = y_i\), the total squared error is given by \(E = \|{\hat{Y} - Y}\|_{2}^2\). You can verify this by substituting the definition of the \(L_2\) norm. Since we want to make \(E\) as small as possible and norms are a measure of distance, this previous expression is equivalent to saying that we want \(\hat{Y}\) and \(Y\) to be a “close as possible.” Note that in general \(Y\) will not be in the range of \(A\) and therefore \(E > 0\).

Consider the following simplified depiction of the range of \(A\); see the following figure. Note this is \(\it not\) a plot of the data points \((x_i, y_i)\).

Illustration of Least Square Regression

From observation, the vector in the range of \(A, \hat{Y}\), that is closest to \(Y\) is the one that can point perpendicularly to \(Y\). Therefore, we want a vector \(Y - \hat{Y}\) that is perpendicular to the vector \(\hat{Y}\).

Recall from Linear Algebra that two vectors are perpendicular, or orthogonal, if their dot product is 0. Noting that the dot product between two vectors, \(v\) and \(w\), can be written as \({\text{dot}}(v,w) = v^T w\), we can state that \(\hat{Y}\) and \(Y - \hat{Y}\) are perpendicular if \({\text{dot}}(\hat{Y}, Y - \hat{Y}) = 0\); therefore, \(\hat{Y}^T (Y - \hat{Y}) = 0\), which is equivalent to \((A{\beta})^T(Y - A{\beta}) = 0\).

Noting that for two matrices \(A\) and \(B, (AB)^T = B^T A^T\) and using distributive properties of vector multiplication, this is equivalent to \({\beta}^T A^T Y - {\beta}^T A^T A {\beta} = {\beta}^T(A^T Y - A^T A {\beta}) = 0\). The solution, \({\beta} = \textbf{0}\), is a trivial solution, so we use \(A^T Y - A^T A {\beta} = 0\) to find a more interesting solution. Solving this equation for \({\beta}\) gives the \(\textbf{least squares regression formula}\):

\[ {\beta} = (A^T A)^{-1} A^T Y \]

Note that \((A^T A)^{-1}A^T\) is called the pseudo-inverse of \(A\) and exists when \(m > n\) and \(A\) has linearly independent columns. Proving the invertibility of \((A^T A)\) is outside the scope of this book, but it is always invertible except for some pathological cases.

< 16.1 Least Squares Regression Problem Statement | Contents | 16.3 Least Squares Regression Derivation (Multivariable Calculus) >