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A rare population is generally defined as a small proportion of a total population that possesses one or more specific characteristics. Examples include billionaires; people with a certain illness, such as gall bladder cancer; or employees in a highly technical occupation. Although the literature offers no precise definition of rare or small in this context, researchers have proposed proportions of .10 or less to identify rare populations. (When this proportion is larger, standard sampling techniques can usually be used efficiently.) In addition, sampling frames are often nonexistent or incomplete for most rare populations. Although researchers can use convenience sampling (e.g. snowball samples) to study rare populations, most efforts in this area have focused on probability sampling of rare populations. The costs and benefits of the ...

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