Nugroho, Mulyanto, and Yusuf Eko Nurcahyo. 2019. “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 9 (5):95-103. https://econjournals.org.tr/index.php/irmm/article/view/8091.