Sampling frame

Both simple random samples and stratified random samples require a complete list of all individuals in the target population. This list might be obtained from an electoral roll or some other publicly available list and is called a sampling frame.

Cluster sampling

Although there are many advantages to simple random sampling, a different sampling scheme called cluster sampling is sometimes used.

Accuracy of cluster sampling

When individuals in the same cluster tend to be more similar than individuals in different clusters, the estimates that are obtained from cluster sampling are more variable (and hence less accurate) than the corresponding estimates from a simple random sample of the same size.

The ordinary formula for the standard deviation of a sample mean will overestimate its accuracy if the mean is obtained by cluster sampling.