Although the calculation meth od of the average value is simple but its meaning is recondite.
平均值的计算方法虽然十分简单，但是，它的内涵却并不是十分简单。
The purpose of OD investigations is to make statistical estimation of the average value of the samples based on the OD data obtained.
OD调查的目的是利用调查所得的OD数据对样本总体的均值作出统计推断。
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