Methodological Note 6: Sample Size and Sampling Strategies in Qualitative Research – How Many Participants Are Enough?

2026-03-11

Claudia Troncoso-Pantoja¹, Patricio Pérez-Armijo²,  Eduard Baladia3,4,5

¹Centro Interdisciplinar de Investigación en Educación y Desarrollo (CIEDE-UCSC). Universidad Católica de la Santísima Concepción, Chile.

²Facultad de Ciencias de la Salud, Universidad Isabel I, Burgos, España.

3Departament de Ciències Experimentals i Metodologia Científica (CEMC), Facultat de Ciències de la Salut i el Benestar, Universitat de Vic, Vic, España.

4Evidence-Based Reserach Network (EBR Network), Bergen, Norway.

5Centro de Análisis de la Evidencia Científica, Área de Gestión del Conocimienti, Academia Española de Nutrición y Dietética, Pamplona, España.

*ctroncosop@ucsc.cl

One of the main challenges faced by qualitative researchers is criticism regarding the small number of participants. Does this constitute a reason to dismiss qualitative research? The answer is no. Qualitative research is characterized by reflexive subjectivity and flexibility, where scientific rigor is not determined by statistical criteria but by the credibility, reliability, confirmability, and transferability of the data (1,2).

Types of Sampling

Qualitative research does not aim to generalize results but to achieve in-depth and interpretive understanding of the phenomenon (3). Therefore, it relies on non-probabilistic sampling techniques (4):

  • Purposive sampling: deliberate selection of participants based on criteria defined by the study objectives.

  • Convenience sampling: selection of participants who are easily accessible due to geographic proximity, availability, or low cost.

  • Snowball sampling: initial identification of a small group that facilitates access to additional subjects with relevant characteristics.

Sample Size and Saturation

Sample size is not determined a priori but emerges during the study and is guided by data saturation. Traditionally, this process has been linked to the concept of saturation, understood as the point at which data collection ceases to yield new relevant information for understanding the phenomenon (5).

Two types are distinguished: code saturation, when analysis generates no new analytic categories; and meaning saturation, when a comprehensive understanding of the dimensions attributed to the object of study is achieved (6,7).

Empirical evidence indicates that thematic saturation is typically reached within approximate ranges: 9 to 17 individual interviews, and 4 to 8 focus groups. These numbers should not be interpreted as prescriptive rules, as the required sample size varies according to methodological factors such as sample homogeneity and clarity of objectives (8,9).

Each sampling method offers practical solutions to different recruitment challenges, with specific advantages and limitations. Purposive sampling allows precise selection of participants with key characteristics for the phenomenon, optimizing data depth and relevance; however, its main limitation is subjectivity in selection and lack of statistical representativeness. Convenience sampling is quick, inexpensive, and easy to implement, ideal for exploratory phases, but introduces significant bias and has limited generalizability. Snowball sampling is particularly effective for accessing hard-to-identify or hard-to-reach populations (e.g., marginalized groups or those with specific traits), although it tends to produce homogeneous samples and depends critically on the willingness and social networks of initial participants. The choice among these methods should be based on study objectives, the nature of the target population, and practical considerations regarding feasibility and resources.

Steps in Constructing a Sampling Plan

  • Step 1: Clearly define the research objective and population of interest.

  • Step 2: Assess access to the population (feasibility, available resources).

  • Steep 3: Select the appropriate non-probabilistic sampling technique for the context.

  • Steep 4: Operationalize clear inclusion/exclusion criteria and acknowledge potential limitations.

Conceptual Integration: How Saturation Guides Sample Size in Qualitative Research

For someone unfamiliar with qualitative methodology, it is essential to understand a key difference compared to quantitative studies: sample size is not calculated beforehand but determined during the research, guided by the principle of "saturation." This means participant recruitment continues until new data no longer provide novel or relevant information for the study objectives.

The sampling strategy determines how participants are accessed, but it is the iterative analysis to achieve saturation that defines when recruitment stops. Both processes are interdependent and occur in parallel.

Imagine investigating young adults’ perceptions of sports supplements. Instead of deciding to interview "50 people" from the start, you begin with a small strategically selected group (e.g., 4–5 regular gym users). After analyzing these first interviews, you identify main themes (e.g., "performance motivation," "concerns about side effects").

As you recruit and analyze more participants (using your chosen sampling technique), you monitor whether new themes or perspectives emerge. The point at which new interviews only confirm existing knowledge, without adding new ideas, is called the “saturation point.” At that moment, recruitment stops, as understanding of the phenomenon is considered sufficient and robust.

Final Reflections

Qualitative methodology provides a flexible and rigorous approach to understanding subjective phenomena in depth. The choice of non-probabilistic sampling technique is a strategic decision that should align with study objectives, population characteristics, and practical access considerations.

Sample size is not a fixed number but the result of a dynamic process guided by theoretical saturation. From a quantitative perspective, where sample size and power calculation are essential for statistical generalization, evaluating a design where participant numbers are not predetermined may be challenging. However, qualitative research operates under a different paradigm: its purpose is not to measure frequency but to explore, deeply understand, and theorize about meanings, processes, and contexts.

The saturation criterion, supported by a growing empirical base (8,9), serves a function analogous to power calculation: it ensures sufficient informational depth. Quality assessment should focus on the transparency and systematicity of the process: justification of sampling, rigorous description of iterative analysis, application of explicit criteria for declaring saturation, and depth of findings generated.

Note: This material is for educational purposes. Its application in actual research should be adapted to the specific study design and complemented with other specialized sources.

 

 

References

(1) L Haven T, Van Grootel DL. Preregistering qualitative research. Account Res. 2019; 26(3):229-244. DOI: 10.1080/08989621.2019.1580147

(2) Eldh AC, Årestedt L, Berterö C. Quotations in Qualitative Studies: Reflections on Constituents, Custom, and Purpose. Int. J. Qual. Methods. 2020; 19:1-6. DOI: 10.1177/1609406920969268

(3) Gill SL. Qualitative Sampling Methods. J Hum Lact. 2020; 36(4):579-581. DOI: 10.1177/0890334420949218

(4) Maxwell JA. Why qualitative methods are necessary for generalization. Qual. Psychol. 2021; 8(1): 111–118. DOI: 10.1037/qup0000173

(5) Renjith V, Yesodharan R, Noronha JA, Ladd E, George A. Qualitative Methods in Health Care Research. Int J Prev Med. 2021; 12:20. DOI: 10.4103/ijpvm.IJPVM_321_19

(6) Rahimi S, Khatooni M. Saturation in qualitative research: An evolutionary concept analysis. Int J Nurs Stud Adv. 2024; 6: 100174. DOI: 10.1016/j.ijnsa.2024.100174

(7) Andriotis K. From the Editor: Saturation in qualitative tourism studies. JQRT. 2024; 5(1): 1-6. DOI: 10.4337/jqrt.2024.01.00

(8) Hennink MM, Kaiser BN. Sample sizes for saturation in qualitative research: A systematic review of empirical tests. Soc Sci Med. 2022; 292: 114523. DOI: 10.1016/j.socscimed.2021.114523

(9) Hennink MM, Kaiser BN, Weber MB. What Influences Saturation? Estimating Sample Sizes in Focus Group Research. Qual Health Res. 2019; 29(10): 1483-1496. DOI: 10.1177/1049732318821692