Mohd Zulkifli Hassan, Adi Irfan Che Ani , Nor Rima Muhamad Ariff. “The Correlation Between External Factors and Housing Developers’ Involvement in Affordable Housing: An Importance-Performance Matrix Analysis (IPMA)”. International Journal of Advanced Science and Technology, Vol. 29, no. 7s, Apr. 2020, pp. 375 -82, http://sersc.org/journals/index.php/IJAST/article/view/9451.