In statistics, a modification of the random sample that is particularly useful when obvious heterogeneity exists in the community, area, etc. to be investigated. In such instances a simple random sample may fail to record sufficient replicates of a particular subcategory, or may do so only very inefficiently, so preventing a proper statistical monitoring of variability. In a stratified random scheme sample points are shared among the main types present, then allocated at random within those subcategories.
http://www.custominsight.com/articles/random-sampling.asp Explanation of the stratified random sample technique.
Subjects: Biological Sciences.