“…Until now, there have been various approaches: some of them purely empirical and others based on mathematical theory, (Sandstrom & Tucker 1993), Brockbank et al (1997), Radjai et al (1999), Schollmann (1999), Erikson et al (2002), Kruyt & Rothenburg (2002), Radeke et al (2002), Bagi (2003), Edwards et al (2003), Kruyt (2003), Vargas et al (2003), Goddard (2004), Metzger (2004), Radeke et al (2004), Chan & Ngan (2005), Nicot & Darve (2005), Tighe et al (2005), Youngquist et al (2005), Chan & Ngan (2006), Liu (2006), Ostojic et al (2006), andSnoeijer et al (2006). The theoretical approaches were pioneered by Kruyt & Rothenburg (2002) and Bagi (2003).…”