A purposive sample is a sample selected in a deliberative and non-random fashion to achieve a certain goal. In a focus group, for example, you may want to consciously seek out respondents at both ends of a spectrum (as well as some in the middle) to insure that all viewpoints are adequately represented. You might also preferentially recruit subjects who have the best knowledgeand experience in an area.
In addition to focus group studies, purposive samples are often used in pilot studies. A purposive sample share the same weaknesses as a convenience sample and you will have difficulty making strong quantitative inferences from such a sample.
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