Sxx Variance Formula Link

Because $S_xx$ is the denominator, it represents the spread of your x-values. If $S_xx$ is small (x-values are clustered tightly), the slope becomes very sensitive to changes. If $S_xx$ is large (x-values are spread out), the slope estimate is more stable.

No — that’s ( \sum x_i^2 ). Sxx subtracts the correction term ( (\sum x_i)^2 / n ). Sxx Variance Formula

And because (\barx = \frac\sum x_in), we have (n\barx^2 = \frac(\sum x_i)^2n). Hence: Because $S_xx$ is the denominator, it represents the