H/H0 = a+b(n/Nd) is the first, simple and foremost empirical model called A-P model to estimate monthly mean daily global solar radiation at the location concerned. The regression coefficients a and b depend on geographical and meteorological condition of the given place. Different researchers proposed different values of regression coefficients for different places. Some are claimed universal nature of the coefficients and derived the values.
Since their geographical and meteorological dependable nature the best and simple way to determine a and b is the statistical technique i.e., linear regression equation/ least square line. For this one should have the data of monthly mean daily i) extraterrestrial radiation (H0), ii) measured radiation (H), iii) actual sunshine hours (n) and iv) maximum possible sunshine duration (Nd) of the site(s).