2.21. Clusters versus strata:

(a) With a cluster random sample, do you take a sample of (i) the clusters? (ii) the subjects within every cluster?

(b) With a stratified random sample, do you take a sample of (i) the strata? (ii) the subjects within every stratum?

(c) Summarize the main differences between cluster sampling and stratified sampling in terms of whether you sample the groups or sample from within the groups that form the clusters or strata.