Improved ratio type estimator using two auxiliary variables under second   order approximation
  In this paper, we have proposed a new Ratio Type Estimator using auxiliary information on two auxiliary variables based on Simple random sampling without replacement (SRSWOR). The proposed estimator is found to be more efficient than the estimators constructed by Olkin (1958), Singh (1965), Lu (2010) and Singh and Kumar (2012) in terms of second order mean square error. 
Prayas Sharma and Rajesh Singh
math.ST stat.TH