M. G. M. Khan and  V. D. Prasad and  D. K. Rao,  On Optimum Stratification .   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    February 2014, vol. 87(3). 508 - 512
    [viewed 16 April 2024]. Available from: https://publications.waset.org/pdf/9997676.