The PRESS statistic (Predicted Residual Error Sum of Squares) 
measures how well the regression equation fits the data.  
It is computed by removing the ith data point from the data set, 
computing the regression equation without this data point, predicting 
that point based on the regression equation, then computing the 
residual.  Theoretically is obtained by repeating this process for 
each data point, then summing up the squares of the resulting residuals.