Variance Formula - Sxx

where (s_x^2) is the sample variance of (x).

If (x_i \sim \texti.i.d. N(\mu_x, \sigma_x^2)): sxx variance formula

It measures the total corrected sum of squares for the predictor variable (x). If (x_i) are fixed constants (standard regression assumption), (S_xx) is not a random variable — it has no variance; it’s just a constant. where (s_x^2) is the sample variance of (x)