结论：推导并建立多重分层、整群随机抽样误差的计算公式。
CONCL USION: the calculation formulas of multiple - stratifying and clustering random sampling are theoretically set up.
分层抽样可以减少由简单随机抽样造成的抽样误差。
Stratified sampling can reduce the sampling error associated with simple random samples.
分层抽样可以减少由简单随机抽样造成的抽样误差。
Stratified sampling can reduce the sampling error associated with simple random samples.
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