To introduce sample selection model and its estimation method.

介绍样本选择模型及其估计方法。

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A new method is proposed for sample selection in large data set.

提出了一种大规模数据集的训练样本选择方法。

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The random sample selection and different sample positions ensure the samples to be representative and fair.

计算机随机选择采样点，采样点车车不同，保证了样品的代表性、公正。

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Mr Heckman confronted the sample-selection problem: in terms of the examples just given, for instance, the effect on the results if the workers who answered the questionnaire differed in important ways from those who did not.

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