Stratification

Target populations can be divided into discrete subpopulations, or strata, on which to increase/decrease sample size. A stream network’s elevation could be used to divide the population into elevation strata, allocating an equal number of sites per stratum (likely yielding inclusion probabilities that vary by stratum because the amount of stream length in each stratum likely would vary). Stratification is the process of grouping members of the population into subgroups before sampling. The strata should be mutually exclusive and also collectively exhaustive so that no population element is excluded Stratification consists of dividing the population into subsets (called strata) within each of which an independent sample is selected. The division of a population into parts is known as strata, especially for the purpose of drawing a sample, an assigned proportion of the sample then being selected from each stratum. The process of stratification may be undertaken on a geographical basis, e.g. by dividing up the sampled area into sub-areas on a map; or by reference to some other quality of the population, e.g. by dividing the persons in a town into strata according to gender or into three strata according to whether they belong to upper, middle or lower income groups. The term stratum is sometimes used to denote any division of the population for which a separate estimate is desired, i.e. in the sense of a domain of study. It is also used sometimes to denote any division of the population for which neither separate estimates nor actual separate sample selection is made. (The International Statistical Institute, "The Oxford Dictionary of Statistical Terms", edited by Yadolah Dodge, Oxford University Press, 2003). OECD Source: Statistics Canada, "Statistics Canada Quality Guidelines", 4th edition, October 2003, page 21.