Strata

Clusters

a. All strata are represented in the sample.

a. Only a subset of clusters is in the sample.

b. Best results occur when elements within strata are internally homogenous.

b. Best results occurs with clusters, when clusters are internally heterogenous.

c. Population divided into few subgroups.

c. Population divided into many subgroups.

d. Choice of elements is within subgroups.

d. Random choice of sub groups.