D. . Juliah Ratnaningsih, A. . Saefuddin, and A. . Kurnia, “Stratified-Extended Cox with Frailty Model for Non-Proportional Hazard: A Statistical Approach to Student Retention Data from Universitas Terbuka in Indonesia”, Thail. Stat., vol. 19, no. 1, pp. 208–227, Dec. 2020.