## Linear Regression using Matrices

### Introduction

For points (x_{1}, y_{1}), (x_{2}, y_{2}), ... (x_{n}, y_{n}), the least square regression line can be given by:

**f(x) = b + mx**

which will minimize the sum of the squared error, which are the error in using the regression function f(x) to estimate the true y values

where e_{i} = y_{i} - f(x_{i}) is the error approximating y_{i} .

Let's see if we can set this up as a system of equations and then solve using matrices.

Using our points (x_{1}, y_{1}), (x_{2}, y_{2}), ... (x_{n}, y_{n}) we would have the following system of equations:

Now let's set up an matrix equation. Let:

This gives us the matrix equation: **Y = XA + E**.

We now just need to solve this for **A**.

The solution to least square regression equation **Y = XA + E** is:

The sum of the squared errors (**SSE**) is:

### Example

**Example: **Determine the least squares regression line using a matrics. The price is $x and y is the monthly sales. Then find the sum of the squared errors.

The solution is to work out A using:

** Step 1:** Get the matrices Y and X.

** Step 2:** Work out the X

^{T}X

** Step 3:** Find the inverse of X

^{T}X. (The tutorial about the Inverse is here)

** Step 4:** Find the X

^{T}Y

** Step 5: **Finally, the result

and the Squared Error is

### References & Resources

- http://www.youtube.com/watch?v=Qa_FI92_qo8

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