2.2. Social network model
Social network analysis (SNA) is a kind of sociological research method by models and parameters, Indicators of density, node degree, distance and centrality are always used in order to understand the structure and properties characteristics of the social network (Wasserman and Faust, 1994). The data were obtained from interviews and investigations, and then the data were processed with UCINET software (version 6.237) to calculate the indicators and draw the network structures (Borgatti et al., 2002).