Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In statistics, the residual sum of squares (RSS) is the sum of squares of residuals. It is a measure of the discrepancy between the data and an estimation model. A small RSS indicates a tight fit of the model to the data. In a standard regression model y_i = a+bx_i+varepsilon_i, where a and b are coefficients, y and x are the regressand and the regressor, respectively, and ε is the "error term." The sum of squares of residuals is the sum of squares of estimates of εi.