“…Brown (1968) also proposed a method for determining the time-varying coefficients of a low-order linear model from a higher-order linear plant. During the 1970s, model order reduction was the centre of interest in the field of system and control theory, which resulted in the advent of various reduction approaches (Priestley et al, 1974;Hickin & Sinha, 1975;Shamash, 1975a;Woon & Marshall, 1975;Shamash, 1975b;Hickin & Sinha, 1976).…”