What is the difference between purposeful and purposive sampling?

Introduction

I’m curious about purposeful and purposive sampling because I’m starting to write up the methods section for my first empirical paper.

In my ethics application, I wrote:

“We aim to recruit English-speaking doctors (or other professionals who might coach doctors) who have the lived experience of using videos of actual clinical practice as a coachee, or coach, or both during the learning conversation to obtain both perspectives. We will commence recruiting from two different groups one where the video has been used to review communication skills (e.g. general practice); and the other where video’s emphasis was to review procedural skills (e.g. surgery). Sampling will be informed by the literature and involve opportunistic, snowball-driven recruiting. We anticipate recruiting approximately 24 participants in total.”

I made no mention of purposeful or purposive sampling, but I’m wondering if that’s what I did.

What I did

I had in mind from the beginning that I wanted to recruit a roughly equal number of supervisors and trainees, and I wanted to recruit people who used video for behavioural as well as those who used video for technical skills. I started by approaching the authors of the research I had identified through the literature, and I recruited by snowballing and word of mouth.

“The use of videos of actual clinic practice appears to be limited in Australia and around the world. In order to recruit doctors (or other professionals who coach doctors) with first-hand experience, I (AYH) will identify (through the literature, word-of-mouth, and snowballing) experts, centres and programs who have experience using video reviews for coaching. We will approach these experts, centres and programs with the hope to recruit these experts as coaches into our study. As part of the snowballing, I (AYH) will ask them to suggest others within and outside their programs and centres whom I might also recruit to participate in the study as both coaches and coachees.”

Approximately mid-way through the data collection process, I do recall a point where I thought I was going to have difficulty recruiting enough participants. However, I found in the technical skills group, the use of video was more widespread than I had anticipated.

In the end I recruited 13 “supervisors” and 13 “trainees”, although many of those who told me about their experience of using video as trainees had experience of using video as a supervisor, and many of the supervisors had experience of using video when they were students or trainees. So in that respect, it wasn’t a “clean” sample. Among those 26 interviewees, I also tried to balance the ratio of those who used video for behavioural skills, and those for technical skills.

So was this purposive or purposeful sampling?

What is purposeful and purposive sampling?

This is the Chat GPT answer:

“Purposive Sampling: This is a type of non-probability sampling where the researcher selects units (like individuals, groups of individuals, or data) based on their own judgment. This selection is not random and is based on the characteristics of the units, which the researcher believes are representative of the population. This method is often used when the researcher wants to study a specific characteristic, behavior, phenomenon, or event.

“Purposeful Sampling: This is a broader term used in qualitative research to refer to different types of non-probability sampling methods, including purposive sampling. The key idea here is that the researcher selects units that are ‘purposeful’ or informative for the study. This could include a variety of sampling strategies, such as selecting units that are most accessible, or units that are varied to capture a wide range of perspectives.

“In essence, while both terms refer to non-random selection of units for study, ‘purposive sampling’ is a specific strategy where units are chosen based on their characteristics, while ‘purposeful sampling’ is a broader term that includes various strategies for choosing units that will be most informative for the research.”

After this, I’m still not clear.

What does the literature say?

I found three articles that talk about purposive or purposeful sampling via a google scholar search.(Campbell et al. 2020; Palinkas et al. 2015; Gentles et al. 2015) The older two are from 2015 and the newer one from 2020. This is what they have to say.

Gentles et al

They sampled the literature and consider the term purposeful and purposive as equivalent, also citing other authors such as Patton. In fact Gentles et al cite Yin to say that we are not actually sampling but rather selecting. Yin argues that in qualitative research, the researchers select what type of participants to recruit.

Gentles et al noted that “the arbitrariness and lack of agreement among methods authors regarding what qualifies as purposeful sampling compromises the clarity and utility of this concept… (and that) in light of these ambiguities, we suggest that whenever researchers describe a sampling process as purposeful they should describe what this means in their specific context, rather than simply state that purposeful sampling was employed.” (Gentles et al. 2015, 1779)

The article goes on to explore other aspects of sampling, and raises in my mind the problematic language around sampling in qualitative research – are we importing language from positivist paradigms inappropriately?

Palinkas et al

Palinkas et al use the language purposeful sampling although purposive does appear once in the article without definition. They summarise purposeful sampling and its rationale quite nicely:

“Purposeful sampling is a technique widely used in qualitative research for the identification and selection of information-rich cases for the most effective use of limited resources (Patton 2002). This involves identifying and selecting individuals or groups of individuals that are especially knowledgeable about or experienced with a phenomenon of interest (Cresswell and Plano Clark 2011). In addition to knowledge and experience, Bernard (2002) and Spradley (1979) note the importance of availability and willingness to participate, and the ability to communicate experiences and opinions in an articulate, expressive, and reflective manner. In contrast, probabilistic or random sampling is used to ensure the generalizability of findings by minimizing the potential for bias in selection and to control for the potential influence of known and unknown confounders.

“As Morse and Niehaus (2009) observe, whether the methodology employed is quantitative or qualitative, sampling methods are intended to maximize efficiency and validity. Nevertheless, sampling must be consistent with the aims and assumptions inherent in the use of either method. Qualitative methods are, for the most part, intended to achieve depth of understanding while quantitative methods are intended to achieve breadth of understanding (Patton 2002). Qualitative methods place primary emphasis on saturation (i.e., obtaining a comprehensive understanding by continuing to sample until no new substantive information is acquired) (Miles and Huberman 1994). Quantitative methods place primary emphasis on generalizability (i.e., ensuring that the knowledge gained is representative of the population from which the sample was drawn). Each methodology, in turn, has different expectations and standards for determining the number of participants required to achieve its aims. Quantitative methods rely on established formulae for avoiding Type I and Type II errors, while qualitative methods often rely on precedents for determining number of participants based on type of analysis proposed (e.g., 3–6 participants interviewed multiple times in a phenomenological study versus 20–30 participants interviewed once or twice in a grounded theory study), level of detail required, and emphasis of homogeneity (requiring smaller samples) versus heterogeneity (requiring larger samples) (Guest et al. 2006; Morse and Niehaus 2009; Padgett 2008).” (Palinkas et al. 2015, 534)

They outline in table 1 a number of different purposeful strategies, and categorise them as emphasising similarity, variation or non-specific emphasis. Looking at the list, I have used a number of these phrases, or feel that these phrases could be applied to the way I recruited and selected my participants. I tried to snowball in the hope that I would be able to use a participant as what Malcolm Gladwell would term a “connector”. It would have been nice o have a confirming and disconfirming case or a extreme or deviant case. I think stratified purposeful i.e. the four major subgroupings (combination of supervisor/trainee and behavioural/technical) would be applicable. My selection was also opportunistic/emergent – where, to an extent, I took advantage of circumstances or events to recruit.

Campbell et al

These authors only use the term purposive and are writing in the context of nursing research. They outline what they mean here:

“Purposive sampling is ‘used to select respondents that are most likely to yield appropriate and useful information’ (Kelly, 2010: 317) and is a way of identifying and selecting cases that will use limited research resources effectively (Palinkas et al., 2015).

“Purposive sampling strategies move away from any random form of sampling and are strategies to make sure that specific kinds of cases of those that could possibly be included are part of the final sample in the research study. The reasons for adopting a purposive strategy are based on the assumption that, given the aims and objectives of the study, specific kinds of people may hold different and important views about the ideas and issues at question and therefore need to be included in the sample (Mason, 2002; Robinson, 2014; Trost, 1986).” (Campbell et al. 2020, 653–54)

If I’m reading this correctly, purposive sampling is the opposite of random sampling.

Conclusion/Reflection

I’m starting to think that neither purposive nor purposeful “sampling” is the right way to describe the cohort of participants I recruited to my study. Yin makes the following argument:

“In a like manner, even referring to your case or cases as a “purposive sample” may raise similar conceptual and terminological problems. You may have intended to convey that the “purposive” portion of the term reflects your selection of a case that will illuminate the theoretical propositions of your case study. However, your use of the “sample” portion of the term still risks misleading others into thinking that the case comes from some larger universe or population of like cases, undesirably reigniting the specter of statistical generalization. The most desirable posture may be to state a clear caveat if you have to refer to any kind of sample (purposive or otherwise). (The preferred criteria and terminology for selecting cases, as part of either a single- or a multiple-case study, are discussed later in this chapter under the topic of “case study designs.”) In this sense, case study research directly parallels experimental research: Few if any people would consider that a new experiment should be designed as a sample (of any kind) from a larger population of like experiments—and few would consider that the main way of generalizing the findings from a single experiment would be in reference to a population of like experiments.” (Yin 2018)

It made me also look at the Ethics application form. Here is where the language of “sampling” from positivist paradigms has crept in:

“Describe the research sample and explain the basis upon which this sample was chosen. Include the number and any other relevant demographic characteristics of participants, as well as any eligibility constraints (i.e. inclusion/exclusion criteria). Explain how potential participants will be identified and approached. Who will do this? Refer to [NS 3.1].”

In the end, I will use the phrase selected rather than using “purposive” or “purposeful sampling”.

References

Campbell, Steve, Melanie Greenwood, Sarah Prior, Toniele Shearer, Kerrie Walkem, Sarah Young, Danielle Bywaters, and Kim Walker. 2020. ‘Purposive Sampling: Complex or Simple? Research Case Examples’. Journal of Research in Nursing 25 (8): 652–61. https://doi.org/10.1177/1744987120927206.

Gentles, Stephen, Cathy Charles, Jenny Ploeg, and K. Ann McKibbon. 2015. ‘Sampling in Qualitative Research: Insights from an Overview of the Methods Literature’. The Qualitative Report, November. https://doi.org/10.46743/2160-3715/2015.2373.

Palinkas, Lawrence A., Sarah M. Horwitz, Carla A. Green, Jennifer P. Wisdom, Naihua Duan, and Kimberly Hoagwood. 2015. ‘Purposeful Sampling for Qualitative Data Collection and Analysis in Mixed Method Implementation Research’. Administration and Policy in Mental Health and Mental Health Services Research 42 (5): 533–44. https://doi.org/10.1007/s10488-013-0528-y.

Yin, Robert K. 2018. Case Study Research and Applications : Design and Methods. Sixth edition. SAGE Publications.