Cluster sampling

Cluster sampling is a sampling technique used when "natural" but relatively heterogeneous groupings are evident in a statistical population. It is often used in marketing research. In this technique, the total population is divided into these groups (or clusters) and a simple random sample of the groups is selected. The elements in each cluster are then sampled. If all elements in each cluster are sampled, then this is referred to as a "one-stage" cluster design. If a simple random subsample of elements is selected within each of these groups, this is referred to as a "two-stage" design. A common motivation for cluster sampling is to reduce the total number of interviews and costs given the desired accuracy. Assuming a fixed sample size, the technique gives more accurate results when most of the variation in the population is within the groups, not between them.