Sample Size and Sampling Techniques

Determining sample size in qualitative research does not follow fixed numerical rules. Instead, the appropriate sample size depends on factors such as the research question, the theoretical framework guiding the study, the qualitative design selected, the type and depth of data being collected, and the time and resources available. Some qualitative projects may focus on a single case, while others may include a small group of participants. The goal is not to generalize to a larger population but to develop a rich, in-depth understanding of the experiences or phenomena being explored.

A strong qualitative sample is one that supports detailed, nuanced analysis without being so small that important insights are missed. Unlike quantitative research, sample size cannot be determined using statistical calculations or power analyses. Instead, qualitative researchers rely on the principle of saturation. Saturation is reached when additional data no longer produce new information, perspectives, or themes. This concept originated in grounded theory but is now widely applied across qualitative methodologies.

In practice, researchers continue recruiting participants until the data become repetitive and no new insights appear. The number needed to reach saturation varies depending on the complexity of the topic and the richness of the data. A review of empirical studies suggests that saturation is often reached with approximately 9 to 17 individual interviews or 4 to 8 focus groups. Regardless of the final number, thoughtful sampling remains essential because it is very difficult to collect data from an entire population.

Sampling Techniques in Qualitative Research

Qualitative research relies on the thoughtful selection of participants who can provide rich, meaningful insights relevant to the phenomenon being studied. Since the aim is not to generalize findings to a larger population but to understand experiences in depth, sampling techniques focus on selecting individuals who can illuminate the research question. Four major sampling strategies are commonly used: purposive sampling, convenience sampling, theoretical sampling, and snowball sampling.

Purposive Sampling

Purposive sampling, also called purposeful or selective sampling, involves intentionally choosing participants who can provide detailed and relevant information about the topic of interest. This approach allows researchers to gather data from individuals who have lived experience or specialized knowledge that is central to the study. Several variations of purposive sampling exist. Typical case sampling focuses on participants who represent what is considered average or ordinary within a phenomenon. Extreme or deviant case sampling seeks out individuals whose experiences fall outside the norm to better understand unusual patterns. Critical case sampling examines one particularly important example to generate insight that may apply to similar contexts. Maximum variation sampling captures a wide range of diverse perspectives to identify both commonalities and differences within a group. Homogeneous sampling selects participants with similar backgrounds or experiences and is often used for focus groups.

An example of purposive sampling can be seen in a study exploring how newly graduated nurses adapt to their first year of clinical practice. Because the goal is to gain deep insight into the experiences of those who have recently transitioned into the workforce, the researchers deliberately recruit participants who meet specific criteria, such as being licensed within the past twelve months and currently working in acute care settings. These individuals are intentionally selected because they can provide rich, relevant information about the transition experience, making them well suited to address the research question.

Convenience Sampling

Convenience sampling selects participants who are easy to access due to their location, availability, or connection to a researcher or organization. This approach is practical when time, resources, or access are limited. Although convenient, this method has lower credibility because participants are chosen based on availability rather than their ability to provide rich or varied insight. Convenience sampling is commonly used in both qualitative and quantitative studies, especially when working with groups that are already assembled in clinical or community settings. For example, a study investigating maternal health practices would recruit mothers who were already attending local maternity clinics, making participation easy and efficient.

Theoretical Sampling

Theoretical sampling is guided by the emerging analysis rather than predetermined criteria. It is closely associated with grounded theory and involves collecting, coding, and analyzing data simultaneously. As new ideas or categories appear in the data, the researcher decides where to collect additional information and from whom, in order to further develop or refine the emerging theory. Each round of data collection is shaped by what was learned previously. Participants are added in phases as concepts emerged from earlier interviews, allowing the researchers to refine and expand their developing theory.

Snowball Sampling

Snowball sampling is useful when studying populations that are difficult to reach or identify, such as marginalized groups or individuals who may not respond to standard recruitment methods. The researcher begins by connecting with a small number of participants who meet the study criteria and then asks them to recommend or refer others who share similar experiences. This referral process allows the sample to grow gradually, much like a snowball gathering size as it rolls downhill.
For example a study about people with Substance Use Disorder can employ this technique. Because participants were part of hidden or stigmatized networks, referrals from initial participants made it possible for researchers to identify others willing to participate.

Table 7.5: Comparing Sampling Techniques (Intellectual property of Dr. Florriann Fehr/TRU Open Press CC BY-NC-SA 4.0)

Remixed from:

• An Introduction to Research Methods for Undergraduate Health Profession Students by Bunmi Malau-Aduli and Faith Alele (2023), published under a CC BY NC 4.0 license.
  • Chapter 4: Qualitative Research

Media Attributions

• Dr. Fehr [avatar] by Research Assistant Katie Gregson on Canva using Canva AI image creation https://www.canva.com/ai-assistant/ is subject to the Canva Pro Content License.

References