The basic idea in sampling is extrapolation from the part to the whole—from the sample to the population (also called the universe). As a corollary, researchers must choose the sample fairly to represent the population. Methods for choosing samples are designs. Good designs involve the use of probability methods, minimizing subjective judgment in the choice of units to survey. Samples drawn using probability methods are probability samples.

Probability samples should be distinguished from samples of convenience (also called grab samples). A typical sample of convenience comprises the investigator's students in an introductory course. A mall sample often consists of the people willing to be interviewed on certain days at certain shopping centers. This, too, is a convenience sample. The reason for the nomenclature is apparent, and ...

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