In classical approaches, this equation does not lead to analyze statistical properties of a and b parameters. As a result of this restriction temporal variation of these parameters could not be obtained because of least square technique assumptions.
However, statistical properties of these assumptions are managed by considering two successive months as one linear equation, (Sahin and Sen 1998). It is shown that the average coefficient values are not enough to represent the whole variability in the meteorological factors, and therefore, their variance and still better distribution functions should be taken into consideration in future global solar irradiation estimations Sen (2001). A simple successive substitution method is proposed by Sahin and Sen (1998).