Damanik, Evo M.T, et al. “The Effect of Work Discipline on Employee Work Productivity at Perumda Tirtanadi Medan Denai Branch”. Innovative: Journal Of Social Science Research, vol. 3, no. 6, Dec. 2023, pp. 4043-56, http://j-innovative.org/index.php/Innovative/article/view/6641.