The least-squares regression method of cost estimation mathematically determines the best fit line through a field of points. Therefore, to the extent that a line doesn't best describe cost ...

The least-squares criterion is a method of measuring the accuracy of a line in depicting the data that was used to generate it. That is, the formula determines the line of best fit.

This is primarily because there has been no quick and robust fitting method in general use such as the fast Levenberg-Marquardt procedure used for least-squares fitting 3.