About

SS or TSS is the sum of the square deviation scores from the mean for one variable.

It's the error calculation of the no predictor model (The basis model).

Equation

<MATH> \begin{array}{rrll} \text{Total sum of squares (TSS|SS)} & = & \sum_{i=1}^{\href{sample_size}{N}}(\href{predictor}{X}_i-\href{mean}{\bar{X}})^2 & \text{for a predictor variable} \\ (TSS|SS) & = & \displaystyle \sum_{i=1}^{N}(\href{target}{Y}_i - \href{mean}{\bar{Y}})^2 & \text{for an outcome variable} \\ \end{array} </MATH>