Improvement in Estimating Population Mean using Two Auxiliary Variables   in Two-Phase Sampling
  This study proposes improved chain-ratio type estimator for estimating population mean using some known values of population parameter(s) of the second auxiliary character. The proposed estimators have been compared with two-phase ratio estimator and some other chain type estimators. The performances of the proposed estimators have been supposed with a numerical illustration. 
Rajesh Singh, Pankaj Chauhan, Nirmala Sawan, Florentin Smarandache