Table 1.

Purposeful sampling strategies in implementation research

Strategy	Objective	Example	Considerations
Emphasis on similarity
Criterion-i	To identify and select all cases that meet some predetermined criterion of importance	Selection of consultant trainers and program leaders at study sites to facilitators and barriers to EBP implementation (Marshall et al., 2008).	Can be used to identify cases from standardized questionnaires for in- depth follow-up (Patton, 2002)
Criterion-e	To identify and select all cases that exceed or fall outside a specified criterion	Selection of directors of agencies that failed to move to the next stage of implementation within expected period of time.
Typical case	To illustrate or highlight what is typical, normal or average	A child undergoing treatment for trauma (Hoagwood et al., 2007)	The purpose is to describe and illustrate what is typical to those unfamiliar with the setting, not to make generalized statements about the experiences of all participants (Patton, 2002).
Homogeneity	To describe a particular subgroup in depth, to reduce variation, simplify analysis and facilitate group interviewing	Selecting Latino/a directors of mental health services agencies to discuss challenges of implementing evidence- based treatments for mental health problems with Latino/a clients.	Often used for selecting focus group participants
Snowball	To identify cases of interest from sampling people who know people that generally have similar characteristics who, in turn know people, also with similar characteristics.	Asking recruited program managers to identify clinicians, administrative support staff, and consumers for project recruitment (Green & Aarons, 2011).	Begins by asking key informants or well- situated people “Who knows a lot about…” (Patton, 2001)
Extreme or deviant case	To illuminate both the unusual and the typical	Selecting clinicians from state agencies or mental health with best and worst performance records or implementation outcomes	Extreme successes or failures may be discredited as being too extreme or unusual to yield useful information, leading one to select cases that manifest sufficient intensity to illuminate the nature of success or failure, but not in the extreme.
Emphasis on variation
Intensity	Same objective as extreme case sampling but with less emphasis on extremes	Clinicians providing usual care and clinicians who dropped out of a study prior to consent to contrast with clinicians who provided the intervention under investigation. (Kramer & Burns, 2008)	Requires the researcher to do some exploratory work to determine the nature of the variation of the situation under study, then sampling intense examples of the phenomenon of interest.
Maximum variation	Important shared patterns that cut across cases and derived their significance from having emerged out of heterogeneity.	Sampling mental health services programs in urban and rural areas in different parts of the state (north, central, south) to capture maximum variation in location (Bachman et al., 2009).	Can be used to document unique or diverse variations that have emerged in adapting to different conditions (Patton, 2002).
Critical case	To permit logical generalization and maximum application of information because if it is true in this one case, it’s likely to be true of all other cases	Investigation of a group of agencies that decided to stop using an evidence-based practice to identify reasons for lack of EBP sustainment.	Depends on recognition of key dimensions that make for a critical case. Particularly important when resources may limit the study of only one site (program, community, population) (Patton, 2002)
Theory-based	To find manifestations of a theoretical construct so as to elaborate and examine the construct and its variations	Sampling therapists based on academic training to understand the impact of CBT training versus psychodynamic training in graduate school of acceptance of EBPs	Sample on the basis of potential manifestation or representation of important theoretical constructs. Sampling on the basis of emerging concepts with the aim being to explore the dimensional range or varied conditions along which the properties of concepts vary.
Confirming and disconfirming case	To confirm the importance and meaning of possible patterns and checking out the viability of emergent findings with new data and additional cases.	Once trends are identified, deliberately seeking examples that are counter to the trend.	Usually employed in later phases of data collection. Confirmatory cases are additional examples that fit already emergent patterns to add richness, depth and credibility. Disconfirming cases are a source of rival interpretations as well as a means for placing boundaries around confirmed findings
Stratified purposeful	To capture major variations rather than to identify a common core, although the latter may emerge in the analysis	Combining typical case sampling with maximum variation sampling by taking a stratified purposeful sample of above average, average, and below average cases of health care expenditures for a particular problem.	This represents less than the full maximum variation sample, but more than simple typical case sampling.
Purposeful random	To increase the credibility of results	Selecting for interviews a random sample of providers to describe experiences with EBP implementation.	Not as representative of the population as a probability random sample.
Nonspecific emphasis
Opportunistic or emergent	To take advantage of circumstances, events and opportunities for additional data collection as they arise.		Usually employed when it is impossible to identify sample or the population from which a sample should be drawn at the outset of a study. Used primarily in conducting ethnographic fieldwork
Convenience	To collect information from participants who are easily accessible to the researcher	Recruiting providers attending a staff meeting for study participation.	Although commonly used, it is neither purposeful nor strategic