Nugroho, Mulyanto, and Yusuf Eko Nurcahyo. “Investment Decision Making: The System Performance of the Portfolio of Shares on Index Business 27 (2017-2018) Using the Model of Constant Correlation”. International Review of Management and Marketing, vol. 9, no. 5, Sept. 2019, pp. 95-103, https://econjournals.net.tr/index.php/irmm/article/view/8091.