Sxx Variance Formula →

Under the :

If (S_xx) is random (random (x)), the of (\hat\beta 1) involves the expectation of (1/S xx). But conditionally on (x), (S_xx) is constant. 5. Small-Sample Correction If (\sigma_x^2) is unknown, replace with (\hat\sigma x^2 = S xx/(n-1)): 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. Under the : If (S_xx) is random (random

Thus: