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Purposive sampling is one technique often employed in qualitative investigation. With a
purposive non-random sample the number of people interviewed is less important than the criteria
used to select them. The characteristics of individuals are used as the basis of selection, most
often chosen to reflect the diversity and breadth of the sample population.
However, there are different approaches to purposive sampling some of which focus on different
aspects of the sample members, cases are chosen because they are considered more extreme, for
example. One form of purposive sampling is ‘theoretical sampling’, developed from the
‘grounded theory’ approach (Glaser and Strauss, 1967). The term ‘grounded theory’ expresses the
idea that theory is generated, through an iterative process, involving the continual sampling,
collection and analysis of data to inform the next stage of the sample design, until ‘theoretical
saturation’ is achieved; that is, no new ideas or theories emerge. The iterative nature of the
theoretical sample design is important. It gives the researcher the opportunity to analyse the data
as the sampling progresses and means that the researcher can add to or change the emphasis of the
sample design, and in doing so ensure robustness of the theories generated. It is therefore valuable
to have considered the analysis technique early on in relation to the qualitative sampling strategy.
Sometimes theoretical hypothesis generation is not the primary aim of the research. Where the
sample population is clearly defined, such as when testing already operational survey questions,
and where resource and time constraints are in place, then a more constrained purposive sampling
strategy can be devised that avoids iteration and does not necessarily achieve saturation, on the
grounds of diminishing returns.
Whatever approach is used, some advance knowledge of the population under investigation is
necessary when carrying out purposive sampling.
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A decision will be required as to the sample selection criteria. That is, what characteristics will
need to be reflected in the sample population to address the research question. The decision on
which criterion to use will be informed by the policy advisor and other subject specialists, as well
as a review of the current literature. The researcher will need to know whether particular sub-
groups need to be included to ensure breadth. The criteria used may be based on demographic
characteristics or behaviours or attitudes, and will need to be prioritised if purposive sampling is
to be employed. This is partly influenced by the fact that qualitative research is often, but not
always, based on a relatively small number of cases so it may not be possible to include all of the
sample criteria in the sample design. Some criteria may be considered more important than others
in relation to the research objectives.
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The intensity and therefore the length of the qualitative interview will also impact on the design
of the qualitative sampling strategy and the decision of sample size. Longer interviews may
provide more data than shorter interviews. A decision may be taken, depending on the nature of
the study, to conduct a larger number of shorter interviews or a smaller number of longer
interviews.
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A feature of qualitative sampling is this fact that the number of cases sampled is often small. This
is because, as mentioned earlier, a phenomenon only need appear once to be of value. There is no
need for scale as there is no need for estimates of statistical significance. Furthermore, because