Amer Ibrahim Falah Al-Omari,  New Product-Type Estimators for the Population Mean Using Quartiles of the Auxiliary Variable.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    January 2009, vol. 25(1). 16 - 20
    [viewed 28 January 2022]. Available from: https://publications.waset.org/pdf/6014.