MATLAB: How to interpret an answer given by the “regress” function

Question

Hello! 🙂

I am trying to link two sets of data through regression. Multiple linear regression, to be more precise. The general regression equation is this:

    y=a0+a1*x1+a2*x2+a3*x3+a4*x4 (1)

So far so good. I have the y matrix, as a 60×1 double and the x matrix, as a 60×4 double. I use the following command:

    b=regress(y,x)

And the answer that I get is this:

    b=0.0073, 0.0074, 0.0071, 0.107

I have two questions:

1. The regression function (1) has 5 coefficients; but here I get only 4. Where am I mistaking?
2. Which one of these four coefficients stored in the b variable goes to which of the ax coefficients from the function (1)?

Thank you!

Answer 1

Your design matrix should be 60x5, not 60x4, you need a column of ones for the constant term, a0.

So start by doing the following:

 x = ones(60,5);

then fill columns 2 to 5 with your x1, x2, x3, and x4 predictors.