The blank can be filled with SST.
The equation can be expressed as:
\[ SST = SSR + SSE \]
where:
- SST (Total Sum of Squares) measures the total variation in the dataset.
- SSR (Regression Sum of Squares) measures the variation explained by the regression model.
- SSE (Error Sum of Squares) measures the variation that is not explained by the model (i.e., the error).