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