1 Introduction  
2 HT Estimator and Its Drawback  
3 Improved HT Estimator  
3.1 Properties of the IHT Estimator  
3.2 The Choice of $K$  
Algorithm 1 The Choice of $K$  
4 Extension to the Ratio Estimator  
4.1 Improved Ratio Estimator  
4.2 Properties of the Improved Ratio Estimator  
5 Numerical Studies  
5.1 Simulations  
5.2 Real Example  
6 Concluding Remarks