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This lesson will explain cluster samples.
Source: Map of Minnesota, Creative Commons: http://en.wikipedia.org/wiki/File:Minnesota_population_map_cropped.png
A sampling method where the population is separated into groups, typically geographically, and a random selection of clusters is made. Each individual in the cluster becomes part of the sample.
Smaller subgroups of the population, not necessarily similar in any way besides all being together in one place, making the individuals easier to sample together.