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Table 1 Purposeful sampling strategies by Patton (2002), adapted by Suri (2011)

From: The use of purposeful sampling in a qualitative evidence synthesis: A worked example on sexual adjustment to a cancer trajectory

Purposeful sampling strategy Purpose (Patton, 2002) Purpose in qualitative evidence synthesis (Suri, 2011)
Extreme of deviant case sampling Learning from highly unusual manifestations of the phenomenon of interest Focusing on how things should be or could be, rather than how things are Suitable for realist syntheses
Intensity sampling Information-rich cases that manifest the phenomenon intensely, but not extremely, such as good students/ poor students, above average/below average. To develop a comprehensive understanding of the phenomena that is been researched in the synthesis
Maximum variation sampling identifying key dimensions of variations and then finding cases that vary from each other as much as possible. To identify essential features and variable features of a phenomenon among varied contexts
Identifies important patterns that cut across variations To construct an holistic understanding of the phenomenon
Homogenous sampling Picking a small, homogeneous sample. Reduces variation, simplifies analysis, facilitates group interviewing To overcome the critique of “mixing apples and oranges”:i.e. to overcome the epistemological incommensurability of different qualitative methods
  To describe some particular subgroup in-depth
  Suitable for participatory syntheses
Typical case sampling Illustrates or highlights what is typical, normal, average To study how common themes recurring in the published literature might be related to the relative strengths and weaknesses of the typical methodologies or theories underpinning the typical studies
Critical case sampling Permits logical generalization and maxi-mum application of information to other cases To assist stakeholders in making informed decisions about the viability of a certain innovation
Snowball sampling Seeking information from key informants about details of other information-rich cases in the field To identify studies that are highly valued by different stakeholders
  To identify studies outside the academic mainstream
Criterion sampling Selecting all cases that meet some predetermined criterion of importance To construct a comprehensive understanding of all the studies that meet certain pre-determined criteria
Theoretical sampling Selecting cases that represent important theoretical constructs about the phenomenon of interest Research synthesis who employ constant comparative methods or grounded –theory approaches
Confirming sampling Selecting cases that are additional examples that fit already emergent patterns; these cases To advocate a particular stance for ethical, moral and/or political reasons
  Suitable for openly ideological synthesis
Disconfirming sampling Selecting cases that do not fit. They are a source of rival interpretations as well as a way of placing boundaries around confirmed findings To shake our complacent acceptance of popular myths and generalizations in a field
Stratified purposeful sampling Sampling within samples where each stratum is fairly homogeneous To examine variations in the manifestation of a phenomenon as any key factor associated with the phenomenon is varied. In a research synthesis, this factor may be contextual, methodological, or conceptual.
Opportunistic sampling Adding cases to a sample to take advantage of unforeseen opportunities after fieldwork has begun To be used in a research area which is at its exploratory stage or when the synthesis does not have an insider status in the relevant field of research
  Suitable to participatory syntheses where the synthesis purpose evolves in response to the changing needs of the participant co-synthesists
Purposeful random sampling Adds credibility to sample when potential purposeful sample is larger than one can handle. Reduces judgment within a purposeful category To locate most of the primary research reported on a topic and then randomly select a few reports from this pool for in-depth discussion
Sampling politically important cases Selecting a politically sensitive site or unit of analysis To gain attention of different stakeholders and the synthesis findings get used.
  Suitable for synthesis of hot topics, in which several stakeholders are interested
Convenience sampling Involve selecting cases that are easy to access and inexpensive to study Not a recommendable technique, because its neither purposeful, nor strategic
Combination or mixed purposeful sampling To use a combination of two or more sampling strategies to select evidence that adequately addresses their purpose To facilitate triangulation and flexibility in meeting the needs of multiple stakeholders