Retnawati, Berta Bekti, and Nuryakin Nuryakin. “Developing Salesperson Performance: The Role of Customer Encountering Competence Portfolio, Relational Capital and Service Excellent Customer Heterogeneity”. International Review of Management and Marketing, vol. 6, no. 4, Oct. 2016, pp. 891-7, https://econjournals.com.tr/index.php/irmm/article/view/3093.