将数据缺失方式划分为随机缺失，非随机缺失与完全随机缺失，这对于合理编写缺失数据推估程序，正确选用估算方法有很重要的意义。

It is very important to divide the missing data into three manners: missing at random (MAR), missing not at random (MNAR) and missing completely at random (MCAR).

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如果数据的缺失完全是随机的，丢弃不完整的回答虽然会减少估计的精度但是不会产生偏离。

If the data deficiency is totally random, then only the precision will be reduced while deviation will not be caused.

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