National Council of Teachers of Mathematics
The Math Forum: Probability and Statistics
This page is a math home page that includes information on all topics of probability and statistics.
Wikimedia
Wikipedia: Cluster Sampling
An encyclopedia entry on cluster sampling details what it is, how it is different from stratified sampling, and provides a discussion on geographical cluster sampling.
University of Glasgow
Statistics Glossary: Cluster Sampling
Contains a definition for what cluster sampling is, when it is used, and also provides an example situation for when it would be used.