The number of strata to use is a matter of judgment, but experience suggests the use of no more than eight.
The logic behind proportionate sampling is simple. Strata with larger relative sizes are more influential in determining the population mean, and these strata should also exert a greater influence in deriving the sample estimates.
A common form of cluster sampling is area sampling, in which the clusters consist of geographic areas such as counties, housing tracts, or blocks.
A major issue related to Internet sampling is representativeness because of lack of computer ownership and Internet access in many households in the United States.