If we know the mean and standard deviation for x and y, along with the correlation (r), we can calculate the slope b and the starting value a with the following formulas: The slope of the least-square...If we know the mean and standard deviation for x and y, along with the correlation (r), we can calculate the slope b and the starting value a with the following formulas: The slope of the least-squares regression line is the average change in the predicted values of the response variable when the explanatory variable increases by 1 unit.