# multiple regression coefficients - Standard error of intercept

I am implementing an R-type summary() function in python with the restriction to exclude use of scientific libraries. (assignment) I found this https://www.nd.edu/~rwilliam/stats1/x91.pdf material explaining the calculation of standard errors of coefficients. As I already had a multiple linear model coded with gradient descent, I was able to code the standard error of coefficients, except the intercept. I just do not understand how to calculate $$S{_b}_0$$ based on the below equation: I can calculate $$R^{2}{_X}_k{_G}_k$$ for k>0, but not for k=0

The following code piece works already, but I need something for the (z=-1) below, which would fill the coeff_stderr element:

        for z in range(len(X)):
xk_vector = get_matrix_column(X, z)
var_xk = variance(xk_vector)
RRxk = coefficient_of_determination_Xk(X, z, num_iters, alpha)
coeff_stderr[z+1] = RSE / sqrt((1-RRxk)*var_xk*(len(X)-1))


what is $$X_0$$? The intercept term - the column with 1s? Then it's variance is 0, and I would divide with 0...

• @Isvan Orosz you might find this useful stats.stackexchange.com/questions/439966/… – PsychometStats Dec 9 '19 at 22:24
• As I understand you are suggesting a matrix inversion along the line? I can't do that the moment with my program. – István Orosz Dec 10 '19 at 14:13
• Yes, you will need to invert covariance matrix to calculate parameters – PsychometStats Dec 10 '19 at 14:22
• if you check out the formula in the linked .pdf, it does not mention inverting covariance matrix. And as I mentioned, the formula works for the coefficients, except the intercept. My solution coefficient_of_determination_Xk() works with gradient descent. – István Orosz Dec 10 '19 at 14:59
• You don't need to invert the matrix: you only need to compute one entry in its inverse. (That's where the formula you quote comes from, by the way: it gives diagonal entries of the inverse.) – whuber Dec 10 '19 at 19:02

        # add intercept term to X (to the left)