Wirjosudirdjo, S. “Perluasan Mengenai Koefisien Realibilitas Dari Kuder-Richardson (KR-20)”. Journal of Mathematical and Fundamental Sciences, vol. 8, no. 2, Feb. 2019, pp. 53-66, https://journals.itb.ac.id/index.php/jmfs/article/view/9684.