Khan, Lakhkar, and Javid Shabbir. “Improved Ratio-Type Estimators of Population Mean In Ranked Set Sampling Using Two Concomitant Variables”. Pakistan Journal of Statistics and Operation Research 12, no. 3 (September 1, 2016): 507-518. Accessed October 2, 2025. https://pjsor.com/pjsor/article/view/1271.