5、多阶段抽样

　　多阶段抽样，也称为多级抽样，是指在抽取样本时，分为两个及两个以上的阶段从总体中抽取样本的一种抽样方式。其具体操作过程是：第一阶段，将总体分为若干个一级抽样单位，从中抽选若干个一级抽样单位入样；第二阶段，将入样的每个一级单位分成若干个二级抽样单位，从入样的每个一级单位中各抽选若干个二级抽样单位入样……，依此类推，直到获得最终样本。

　　多阶段抽样区别于分层抽样，也区别于整群抽样，其优点在于适用于抽样调查的面特别广，没有一个包括所有总体单位的抽样框，或总体范围太大，无法直接抽取样本等情况，可以相对节省调查费用。其主要缺点是抽样时较为麻烦，而且从样本对总体的估计比较复杂。

[此贴子已经被作者于2008-7-5 10:07:54编辑过]

是第一阶段先抽取行业，然后第二阶段再从行业中抽取具体企业吗。

还请讲细点，谢谢，呵呵。

Multistage sampling is a complex form of cluster sampling. Using all the sample elements in all the selected clusters may be prohibitively expensive or not necessary. Under these circumstances, multistage cluster sampling becomes useful. Instead of using all the elements contained in the selected clusters, the researcher randomly selects elements from each cluster. Constructing the clusters is the first stage. Deciding what elements within the cluster to use, is the second stage. The technique is used frequently when a complete list of all members of the population does not exist and is inapproprate.

Probability-proportional-to-size sampling is a type of multistage cluster sampling. In this method, the probability of selecting an element in any given cluster varies inversely with the size of the cluster.

Although cluster sampling and stratified sampling bear some superficial similarities, they are substantially different. In stratified sampling, a random sample is drawn from all the strata, where in cluster sampling only the selected clusters are studied, either in single stage or multi stage.