“…When X 1 = X 2 such as for variance estimation in (Muneer, Khalil, Shabbir, and Narjis, 2018), this is a particular case of the additive parametric ratio estimator introduced just above. The third one is related to (Yasmeen, Amin, and Hanif, 2015;Adichwal, Sharma, and Singh, 2017). See for a comparison of this kind of estimators.…”