It is shown that analysis of the goodness of a fit using only the coefficient of determination R2 or the mean average deviation is not enough and should be accompanied by a statistical study of the behavior of the residuals. This study should not be Visual but using proper statistics. Different techniques such as normal probability plots, half-normal probability plots, detrended probability plots, rankits and Durbin-Watson are presented and illustrated with examples from the area of fluid behavior and fluid phase equilibrium.