Sxx

Sxx is a symbolic label that appears in multiple disciplines and does not have a single, universal definition. Its meaning depends on the context.

In statistics and data analysis, Sxx denotes the sum of squared deviations of x from its mean: Sxx = sum_i (x_i - x̄)^2. It is a key component in simple linear regression: the slope β1 = Sxy / Sxx, where Sxy = sum_i (x_i - x̄)(y_i - ȳ). Sxx is related to the variance of x by Sxx = (n-1)s_x^2.

In time series and signal processing, Sxx commonly denotes the auto-power spectral density (auto-spectrum) of a process X, i.e., the Fourier transform of its autocovariance. It characterizes how power is distributed over frequency; estimates of Sxx are obtained via periodograms or PSD estimators.

In physics and engineering, Sxx may appear as the xx component of a second-order tensor, such as a stress or strain tensor, depending on the author's convention. The precise interpretation varies by discipline, and more common symbols include σxx for stress or εxx for strain.

Thus, Sxx is a context-dependent symbol with at least these principal uses, and clarity comes from defining it in the given mathematical model or analysis.