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1·The reasons there are two main expressions for population and sample is that one is biased estimator and the other is not.

“母体”与“样本”之所以有两种表达公式，是因为一个是有偏估计，另一个则不是。

2·We give the approximation formulas of the variance of the sample ratio estimator for a population mean and its asymptotically non-biased estimator in multi-stage sampling.

讨论多阶段抽样比率估计及其样本量选择的问题，给出了比率估计抽样均方误差的近似公式及其渐近无偏估计的公式。

3·Instead of using the homomorphic filter which is used to get rid of multiple noise and is a biased estimator, this algorithm translates speckle into locally stationary additive white noise.

该算法不同于有偏的去除乘性噪声的同态滤波算法，而是将噪声转化为局部平稳的加性白噪声。