【多选题】Which of the following statement about cluster sampling is/are true?
A.
Cluster sampling is used where the population is divided into already existing groupings.
B.
Clusters are usually formed by geographical locations.
C.
Cluster(s) is selected randomly from the population.
D.
In each selected cluster, simple random sampling or systematic sampling can be used for selecting sample members.
E.
In each selected cluster, even census can be used for selecting sample members.
F.
Cluster sampling can be viewed as a compromise between cost and representativeness, especially when research scales are large and travelling costs for fieldwork are high.