CK-12 Foundation
Ck 12: Statistics: Sampling Methods
[Free Registration/Login may be required to access all resource tools.] Decide how to choose a sampling of a population to produce reliable statistical results.
University of Glasgow
Statistics Glossary: Stratified Sampling
An online statistics glossary which contains the definition for stratified sampling. Lists reasons for using stratified sampling instead of simple random sampling and gives an example situation in which stratified sampling may be used.
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.