"Sex adult product factory" Cluster sampling, the sample is a group of units to extract a group of units. Cluster sampling has two key steps: ① "Sex adult product factory" overall homogeneity is divided into completely separate smaller subset. ②"Sex adult product factory" constitute a randomly selected subset of the sample. If the investigator in a subset of observations drawn all the units, we have a whole group of samples. If the child focus on the ballot then the probability of observing the way to extract some of the units, we have a secondary cluster samples. Stratified and cluster sampling should be generally divided into subsets completely independent of each other. The difference between them is that the sample is stratified sampling concentrated extract from each child, and cluster sampling is to extract part of the subset. Geographical area is a typical way of sampling cluster sampling.
[This paper edited by "Sex adult product factory http://www.zhsextoy.com/products-detail.asp?cpid=280", please indicate the source !]